Are sentences in german different cases just because of prepositions? i'm really confused on how to know when its nominative, accusative or dative. .

Yes, they can be (but are not always so). There are sets of prepositions that require the use of certain cases. There are also other sets of prepositions that induce multiple possible cases (and you need to learn to distinguish which to use).

It can be confusing in the beginning, simply because English does not have so many forms for which the case distinction matters. But luckily there are some learnable rules for German.

The prepositions that always require the Accusative:

durch, fuer, ohne, gegen, um

always Dative:

aus, bei, mit, nach, von, zu, seit.

and Genitive:

trotz, während, statt, jenseits, innerhalb, ...

The nominative is easy and usually does not bind with prepositions, anyway.

Finally, the are some trickier prepositions that bind with either Dative, or Accusative:

an, auf, hinter, in, neben, ueber, unter, vor, zwischen, entlang

For the above ones you need to distinguish the correct case, and it requires a bit of exercise. The rule is that if in the given case you can ask the question "where TO" the preposition will induce the Accusative. For example: "Ich lege das Buch auf den Tisch." But if you can ask "whereabout?" the preposition induces the Dative. Example: "Das Buch liegt auf dem Tisch."

Have fun learning!

sarah gibt den katzen das futter.   sarah flutter

thomas isst das essen.   er

die kinder kaufen die geschenke   die kinder kaufen

sie spielen mit dem freund.   sie spielen mit

wir kaufen der schwester ein buch.   wir buch.