不定式的主动时语态有三种形式：1.一般式。to do
2.进行式。to be doing
3.完成式。to have done
一.不定式的一般式所表示的动作与谓语动词所表示的动作同时发生或发生在谓语动词所表示的动作之后。例句：
1.I have a composition to write.
2.I want to buy a car.
二.不定式的进行式所表示的动作与谓语动词所表示的动作同时发生。例句：
1.Tom pretended to be sleeping when his mother came in.
2.Mary seemed to be worrying.
三.不定式的完成式所表示的动作发生在谓语动词所表示的动作之前。例句：
1.I am sorry to have kept you waiting so long.
2.We are all very glad to have seen your performance.
