Adjectives describe nouns; they specify the way something “is” (What is the weather like? – it´s nice). Predicative adjectives cannot be declined.

Das Wetter ist schön.

The weather is nice.

Adverbs describe verbs; they specify the way something is “done” (How does he swim? – He swims quickly/fast). Adverbs cannot be declined, they don´t change their form. Further adverbs are for example: hier (here), sehr (very), abends (in the evening), …

Heute bleibe ich im Bett.

Today I'll stay in bed.

The verb “sehen” (to see) needs the accusative object, this means article, pronoun and noun need to be in the accusative case.

Willst du den Film sehen?

Do you want to see the movie?

Determiners and nouns together form a nominal group: “ein”(a), “mein” (my).

Die Musik gefällt mir

I like this music.