还没【hái méi】

还没 says something has not happened up to now, and leaves open that it still will.

还没 against 没

没 reports that it did not happen. 还没 reports that it has not happened yet, which is a different claim.

No 了

A sentence with 还没 cannot also take the verbal 了. wǒ我Huán还méi没chī吃le了 is wrong.

呢 at the end is common and adds a note of "still waiting":

过 survives

This says the experience is still ahead, where 我没去过中国 simply says it hasn't happened.

Asking

The answer is 吃了 or 还没.

还没…就 for something that happened too early