题目内容：

-What is behind the door?
-There must be a dog sitting behind it.
sitting behind it,为什么是sitting?而不是sit 或sat?

优质解答

不 用 sat, 因 为 不 是 过 去 时 .
不 用 sit, 因 为 第 三 人 称 单 数 要 用 s: sits.
用 sitting 是 因 为 状 态 正 在 进 行 .
A dog is sitting there.
A dog is sitting behind the door.
There is a dog sitting behind it (the door).
There must be a dog sitting behind it.