Questions & Answers about Спасибо, я уже сыт.
In Russian, the verb to be is normally omitted in the present tense.
So:
- я сыт = I am full
- literally, Russian just says something like I full
This is completely normal. You do use forms of быть in other tenses:
- я был сыт = I was full
- я буду сыт = I will be full
But in the present tense, есть is not used here in standard modern Russian.
Сыт is the short form of the adjective, and that is the normal form used here.
Russian often uses:
- the full form of an adjective when it describes a noun: сытый человек = a full/satisfied person
- the short form when it states someone’s current condition: я сыт = I’m full
So in this sentence, сыт is the natural choice because it describes a current state.