i have a doubt. which is ''tu as faim''. question or sentence. because you are using in same way.

Tu as faim -- You are hungry.
Tu as faim ? -- Are you hungry.

It depends on the intonation; the words don't change.

There are three ways to phrase a question in French:

1) With est-ce que... : Est-ce que tu as faim?
2) With inversion: As-tu faim?
3) With intonation: Tu as faim?

All three are asking the same thing. But with the third option you just use intonation to make it into a question.

-- Chris (not a native speaker).