The declension is the main problem when learning german. Let' s start from the beginning.

All what is said here is explained in a more detailed way in the grammar, for instance here: Deklination.

1) In hundred if not thousand of forums all over the internet you can read that german is difficult, because of the declension

This affirmation is a little bit strange because german is, regarding the declension, a typical language there are very few languages, among them English, Spanish, Swedish and Persian, who don' t decline. But the vaste, vaste majority of languages, Russian, Arabic, Polish, Hungarian decline and there are even a lot of language that have more cases the german, much more even like Finnish. It is therefore nonsense to say that german is difficult because it declines, because almost every language decline.

2) What exactly is declension?

It would be possible, in the sense that it would be understood, to say just

Jim gives apple Maria.

One would understand this sentence because our knowledge of the real world allows us to understand the meaning, it is not possible that "Jim gives Maria to the apple", the only possible meaning is "Jim gives the apple to Maria". But the brain has to work a little bit more if the relationship between Jim, Maria and the apple is not established and if there are many sentences of this type there is a good chance to get lost, most of all in the spoken language. If the sentence is a little bit more complicated comprehension becomes really complicated.

~ I donate bicycle association Jim.

There are two possible meanings. 'I donate the bicycles to the association of Jim' or 'I donate the bicycles of the association to Jim'. It is easy to see that a sentence is easier to understand if the relationship between the objects are established. That' s what the declension does. With the declension we establish the relationship between the object of a sentence. If we want to be accurate we can even say, that there is no language on earth without declension, because every language must establish the relationship between the objects. The difference is, as we will seen soon, that some language, like english, do it with preposition and the other languages, among these german, by changing the form of the article / possessive pronoun / adjective / noun itself.

3) How the different languages establishe the relationship between the objects of a sentence?

Let' s start with the indirect object, the dative, to the girl / dem Mädchen.

To mark un indirect object, a dative, we use the preposition 'to' in english, in german the word or the article changes. Le nominative is 'das Mädchen', but the dative is 'dem Mädchen'.

Obviously the same thing could be done in english, and in old english (the english spoken between the mid-5th century and the mid-12th century) it was actually done that way. The sentence below is an INVENTED EXAMPLE and has nothing to do with old english.

~ I donate applam girlo.

'girlo' would correspond to 'to the girl' and 'applam' would be a direct object, which is not marked nowadays english, the only difference between the nominative and accusative in English is the position in the sentence (The dog sees the horse <=> The horse sees the dog).

If we agree that an -o is to be added in the dative case and -am in the accusative case we can make make any sentence this way.

~ I donate my brothero caram.

The equivalent would be 'I donate a car to my brother.

If we mark the accusative, we can change as well the position of the elements in a sentence.

~ Horse sees dogam <=> ~ Dogam sees horse.

In both cases the dog is seen by the horse. That' s how it works in latin. The position of the objects inside the sentence is irrelevant, you can put them anywhere, because their role is defined by the declension.

It is therefore incorrect to say that german declines and other languages don' t do that. Every language declines, although not the same way. Some, a minority, establish the relationship between the object of a sentence through prepositions, others by changing the words themselves, but every language establish these relationship, otherwise it would be impossible to communicate.


4 ) Personal pronouns

There are a lot of pronouns, personal pronouns (I, you, he / she / it, we etc.) possessive pronouns (my, your, his / her etc.), demonstrative pronouns (this, that, these / those), relative pronouns (The man, who / that..., The book, which / that...) etc. etc. All pronouns point to someone, a sentence like "He is happy" is useless, if I don' t know to whom 'he' refers to. But all the pronouns, except the personal pronouns, have a second function. The possessive pronouns describe to whom something belongs to, the relative pronoun establish a relationship between the main clause and the relative clause etc.. But the personal pronous has only one function, it represents something, stand for something.

Until know we have said, that the relationship between the objects of a sentence is established in English through prepositions. This is true for nouns and the pronouns, except the personal pronouns, the personal pronouns are declined, in other words, they change their form depending on their function inside the sentence. But regarding the personal pronouns in english there is a big difference between English and the other languages, because English is perhaps the only language which doesn't distinguish in the personal pronouns between dative and accusative.

dative
I give him the book.
I give her the book.
I give them the book.

accusative
I see him.
I see her.
I see them.

There is no difference between dative and accusative in English, both are, me, you, him / her, us, you, them. The term "objective pronouns" is only useful in English, in any other language, this term doesn' te exist, because it wouldn' t make any sense if the accusative and the dative have different forms. It is therefore crucial to understand that an accusative object and a dative objects are really two different beasts, although the same personal pronoun is used for them in english.

accusative
I see him.
Ich sehe ihn

dative
I give him the book.
Ich gebe ihm das Buch.

The personal pronoun in english is the same for accusative and dative, but in german the accusative is ihn and the dative is ihm.

It is therefore important to be able to distinguish between an accusative and a dative. We will discuss now three different way to find out wheter an object is an accusative or a dative.

5 ) How to distinguish between a direct object (accusative) and an indirect object (dative)?

There are two different methods to find out wheter an object is an accusative or a dative.

a) Convert the sentence in active voice to a sentence in passive voice

Al transformar una frase en voz pasiva en una frase en voz activa el objeto directo de la frase en voz activa se convierte en el sujeto de la frase en voz pasiva. Una frase está en voz pasiva si el sujeto no es el ejecutor de la acción pero la meta de la acción descrita por el verbo. (Jaime come una manzana (Jaime es el sujeto de la frase y ejecutor de la acción y no la meta, la meta es la manzana) <=> La manzana es comida por Jaime (La manzana es el sujeto de la frase, pero no el ejecutor de la acción. La manzana es la meta de la acción.) El sujeto de la frase rige el verbo: La manzana es comida / Las manzanas son comidas.

voz activa: Jaime da un libro al hombre.
voz pasiva: El libro es dado al hombre por Jaime.

En la frase en voz activa 'un libro' es el objeto directo. En la frase en voz pasiva 'el libro' es el sujeto de la frase. En la frase en voz activa es Jaime quién rige el verbo, en la frase en voz pasiva es el libro que rige el veerbo. Si se pone la frase al plural el verbo también cambia en la frase en voz pasiva.

voz activa: Jaime da unos libros al hombre.
voz pasiva: Unos libros son dados al hombre por Jaime.

'Al hombre' es el objeto indirecto y no sufre ningún cambio en esta transformación.

b) Sustituir los objetos por pronombres personales

Hemos visto ya en 6) que el español, como todas las lenguas aparte del inglés, tiene formas propias para el acusativo y dativo en los pronombres personales. Si sabemos que la / lo / las / los son acusativos y le / les dativos podemos sustituir los sustantivos por pronombres personales. Si se los puede sustituir con la / lo / los / las son acusativos, si se puede sustituirlos con le / les son dativos.

Jaime da un libro al hombre.
Jaime le da el libro. => Sustitución con le. 'Al hombre' es dativo.

Jaime da un libro al hombre.
Jaime lo da al hombre => Sustitución con lo. 'Un libro' es un objeto directo.

Como le, por razones mencionados en 5), también puede ser objeto directo, más vale hacer está sustitución con las formas femininas, que no son ambiguas.

Ayudo a Jaime.
Le ayudo => ambiguo: Le puede ser objeto directo y indirecto.
La ayudo => no es ambiguo: Ayudar exige un objeto directo en español

c) If we transforme a sentence in active voice to a sentence in passive voice the direct object of the sentence in passive voice became the subject of the sentence in passive voice.

active voice: the apple is the direct object, the accusative
John eats an apple.
passive voice: the apple is the subject of the sentence, the nominative
The apple is eaten by John.

(The nominative of a sentence rules the verb. In other words, if you put the sentences to the plural, the verb is in plural as well: The apples are eaten by John. A sentence is in passive voice if the subject is the goal of an action, but not the executor of this action. In the sentence 'John eats an apple' John is the executor of the action. In the sentence 'An apple is eaten by John' the apple is the goal of the action, but not the executor of this action.)

Another example:

active voice: the book is the accusative object
Jim gives a book to the man.

passive voice: the book is the subject of the sentence
A book is given to the man by Jim.

In the sentence in active voice 'the book' is the direct object, the accusative. In the sentence in passive voice 'the book' is the subject of the sentence. In the sentence in active voice John rules the verb, in the sentence in passive voice 'the book' rules the verb. If we put the sentence in plural in the sentence in passive voice we have to modify the verb as well.

active voice: Jim gives some books to the man.
passive voice: Some books are given to the man by Jim.

This is the most accurate form to know what is the accusative and what is the dative of a sentence. The following rule is less accurate, but works in most of the cases.

b) The indirect object is the living being

In english you use the objective pronouns after a preposition and due to the fact, that there is no difference in the form between a direct and an indirect object, it is always the same.

She goes the cinema with him.
They talked about him.

In German things are a little bit more complicated. Some prepositions requiere the dative and other prepositions require the accusative.

mit / with requieres the dative
She goes the cinema with him.
Sie geht mit ihm ins Theater.

über / about requieres the accusative
They talked about him.
Sie sprechen über ihn.

In english we have him in both cases, in German we have ihn or ihm. In other words, if you learn a new preposition in German, you must learn as well the case this preposition requieres.

It can even be more subtle. Some prepositions requires the accusative or the dative depending on whether we talk about a mouvement towards a place or a mouvement in a place.

The bird flies above the garden.
The birds flies over the garden.

In English two different prepositions are used to make the difference. If the bird flies over the garden, he flies from one tree to another tree crossing the garden but if bird flies above the garden he circulates above the garden going nowhere. In German this difference is made with different cases using the same preposition.

über with dative = a mouvement inside a place
The bird flies above the garden.
Der vogel fliegt über dem Garten.

über with accusative = a mouvement towards or over a place
The birds flies over the garden.
Der Vogel fliegt über den Garten

in with dative = a mouvement inside a place
I am in the garden.
Ich bin im (in dem) Garten.

in with accusative = a mouvement towards a place
I go to the garden.
Ich gehe in den Garten.

8) About all the topics mentionned here we will talk once again more in detail in the grammar. Another problem with the declension is the fact that there are different models and the declension depends as well from the words before the noun.

der Mann => des Mannes pero die Frau => der Frau
rote Rosen => die roten Rosen

As you can see the declension is not always the same. The genitive of man / Mann is Mannes, there is a change in the word, but the genitive of woman / Frau is Frau, there is no change at all. The article used are not the same as well, in the case of Mann it is 'des', but in the case of the Frau it is 'der'.

If you look at rote Rosen and die roten Rosen, both are nominative, you can see that without an article it is 'rote' but with an article it is 'roten'. There is no need to understand that, for the moment just watch the videos. We will took about that again later.

One last thing. The children present the declension the way they learnt at school. At school the children are taught that each case has its special interrogative pronoun.

For the accusative you ask with 'wen oder was'.

Ich sehe den Apfel.
Wen sehe ich?
Den Apfel.
I see the apple.
What do I see?
The apple.

For the genitive you ask with 'wessen'.

Ich mag die Farbe des Apfels.
Wessen Farbe gefällt mir?
Die Farbe des Apfels.

I like the colour of the apple.
Whose colour do I like?
The colour of the apple.

For the dative you ask with 'wem'.

Was ich sage ist dem Apfel egal.
Wem ist egal was ich sage?
Dem Apfel.

The apple doesn' t care what I say.
Who doesn' t care what I say?
The apple.

This explanation is completely useless from a linguistic point of view, because that way nobody understands what the declension is about and it is useful to understand that, even if german native speakers know which case to use in german. But if they want to learn a foreign language, it is not enough to speak correctly in their mother tongue. If they want to learn a foreign language a deeper understanding of the declension is needed. The children who made these videos are smart, there is no doubt that one day they will get a deeper understanding of the phenomena, although the school was not very helpful. It would be interesting to discuss about the educational systems all over the world and about their failure, but that is another topic.

Nominativ
Das ist der Apfel
Wer oder was?
Das ist der Apfel

nominative
This is an apple.
What is it?
An apple.

Genitiv
Ich mag die Farbe des Apfels
Wessen Farbe?
Ich mag die Farbe des Apfels

genitive
I like the colour of the apple.
Whose colour do I like?
I like the colour of the apple.

Dativ
Was ich hier erzähle, ist dem Apfel egal.
Wem ist es egal?
Was ich hier erzähle, ist dem Apfel egal.

dative
The apple doesn' t care what I am telling.
Who doesn' t care what I am telling.
The apple doesn' t care what I am telling.

Akkusativ
Ich sehe mir den Apfel an
Wen oder was?
Ich sehe mir den Apfel an.

acusative
I look at the apple.
What do I look?
I look at the apple.