1 you lost the keys. Have you found them?(that) -> have 2. They said something.Did you hear it? (What) Did you hear ...

1, Have you found the keys that you have lost?

⇒That thay cho which, có thể dùng that khi danh từ đi trước chỉ ng, vật.

2, Did you hear what they said?

⇒ What =the things that: những thứ mà.