The articles and the adjective declensions.
In English, there are only the articles "a, an, the", but in German there are "ein, eine, der, die, das". Unlike English, in German the nouns can be masculine (der), feminine (die) or neutral (das). And the adjectives you use for these nouns change based on their article (der, die or das). For example:
A song = ein Lied (neutral)
A nice song = ein schönES Lied
A woman = eine Frau
A nice woman = eine schönE Frau
Then the verbs or prepositions are akkusativ, dativ or genetiv. That means they are used either with objects or other forms. You need to be careful what kind of verb and with what preposition you are using because then the articles would change as a result. For example some verbs are used with objects like the verb "read", etc.
The examples would be so complicated, I can't just type all of them here, sorry. It takes a life-time to learn it all properly, lol. Jk...not that bad though. Practice makes perfect.