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 Äpfel
Wer oder was?
Das sind die Äpfel

nominative
These are apples.
What is it?
These are apples.

Genitiv
Ich mag die Farbe der Äpfel
Wessen Farbe?
Ich mag die Farbe der Apfel

genitive
I like the colour of the apples.
Whose colour?
The colour of the apples.

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

dative (in English it is not a dative)
The apples don' t care what I say.
A qué da igual? Who doesn' t care?
The apples don' t care what I say.

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

accusative
I look at the apples.
What do I look at?
I look at the apples.