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 ist die Erdbeere
Wer oder was?
Das ist die Erdbeere

nominative
This is a strawberry.
What is it?
This is a strawberry.

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

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

Dativ
Was ich hier erzähle, ist der Erdbeere egal
Wem ist es egal?
Was ich hier erzähle, ist der Erdbeere egal

dative (in English it is not a dativ)
The strawberry doesn' t care what I say.
Who doesn' t care?
The strawberry doesn' t care what I say.

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

accusative
I look at the strawberry.
What do I look at?
I look at the straweberry.