What is to be said about the declension has already been said here: 3.2.13 The combinaison ck. Open the video and look at the grammar explanations. The information you find there is an introduction to the topic, the details are a little bit more complicated and will be reconsidered in chapter 9), the declension in german, chapter 10), the pronouns, chapter 13), some prepositions, chapter 16), possessive pronouns etc. etc. The declension plays an important role in German. For the moment we just need to know what it is and have a general idea how it works.

Nominativ
Das sind die Blätter
Wer oder was?
Das sind die Blätter

nominative
Estos son las hojas These are the leaves.
¿Qué son? Who are they?
Estos son las hoja These are the leaves.

Genitiv
Ich mag die Farbe der Blätter
Wessen Farbe?
Ich mag die Farbe der Blätter

genitive
I like the colour of the leaves.
Whose colour?
I like the colours of the leaves.

Dativ
Was ich sage, ist den Blättern egal
Wem ist es egal?
Was ich sage, ist den Blättern egal

dative (in English it is not dative)
The leaves don' t care what I say.
Who doesn' t care?
The leaves don' t care what I say.

Akkusativ
Ich sehe mir die Blätter an
Wen oder was?
Ich sehe mir die Blätter an

accusative
I look at the leaves.
What o I look at?
I look at the leaves.