还没【hái méi】

还没 (or 还没有) goes before the verb and says something hasn't happened up to now, leaving open that it still will.

还没 or 没

没 reports that something didn't happen. 还没 reports that it hasn't happened yet, and expects it to.

No 了

A sentence with 还没 can't also take the verbal 了. 呢 at the end is common and adds a note of still waiting.

• × 我还没吃了 → 我还没吃呢

Asking

The question puts 了没有 or 了吗 after the verb. The answer is the verb with 了, or 还没.

过 stays after 还没. 我还没去过中国 says the experience is still ahead; 我没去过中国 only says it hasn't happened.

还没…就… says one thing happened before another had finished.