(1) 一是表示将来，即表示发生在谓语动作之后的动作。如：
We decided to leave early. 我们决定早点动身。
He asked me to buy him some paper. 他叫我给他买些纸。
We expect him to come in time. 我们希望他能及时来。
(2) 表示与谓语动作同时发生或略先于谓语动作的动作。如：
He seems to be tired. 他似乎累了。
Who heard him say that? 是谁听到他这样说的？
I’m sorry to hear that. 听到这事我很难过。(to hear 略先于am sorry)