到底【dàodǐ】

到底 in a question demands a straight answer – "on earth", "exactly", "so which is it?". It goes after the subject and before the verb. The speaker is impatient or confused.

When the question word itself is the subject, 到底 goes first.

No 吗

到底 needs a real question: a question word, or the verb-不-verb form. It never goes with 吗.

• × 你到底去吗？ → 你到底去不去？
• × 他到底喜欢你吗？ → 他到底喜不喜欢你？

"In the end, after all"

In a statement, 到底 says something finally came about after a long wait or a lot of effort. It often comes with 还是.

到底是 + noun explains something by what it is: "after all, she is a child".