5 ）在 make do, make believe, let drop, let fall, let fly, let slip, let drive, let go off, hear, say, hear tell, leave go of 等固定搭配中，用不带 to 的动词不定式。例如：
    They let go off the rope 。他们松开了绳子。
    John let fly a torrent of abuse at me 。约翰朝我痛骂了一顿。
    I've heard tell of him 。我听说过他。
    Some of the faculty were let go for lack of the students. 由于生源不足，一些教职员工被解雇了。
    6 ）在介词 but, except 之后，如果其前有动词 do 的某种形式，其后不定式一般不带 to ，反之则必须带 to ，表示 “ 不得不，只能 ” 。例如：
    He will do anything except work on the farm.
    There was nothing left for the enemy to do but surrender.
    The spy was both hungry and cold; there was nothing left for him but to give in.
    I had no choice but to wait till it stopped raining.
     下面一些短语是固定搭配，不带 to ：
    can not help but, can not choose but, can not but, do nothing but, have nothing to do but 。例如：
    I can not but admire his courage.
     如果上述句中有 do ， to 省略：
    I did nothing but watch TV last night.
      如果是下面一个固定搭配，就带 to ：
    I have no choice but to give up my idea.

7 ）紧跟在 why 或 why not 之后的动词不定式总是不带 to 。但是，紧跟 who, what,
    which, whether 等连接词后的不定式带 to 。例如：
    Why stand up if you can sit down?
    Why not ask your teacher when you don't understand the meaning?
    You needn ' t decide yet whether to study arts or science.
    4 不定式的完成式和进行式
    1 ）构成
     完成式： to+have done
     进行式： to+be doing
    2 ）用法
     完成式：如果不定式所表示的动作（状态）发生在主要谓语动作之前，那么不定式就要用其完成式。
     进行式：如果主要谓语动作（情况）发生时，不定式所表示的动作正在发生，那么不定式就要用其进行式。例如：     She feels relaxed to have finished writing her thesis before the deadline.
    The Vikings are believed to have discovered America.
    When you called me last night, I happened to be working on the computer.
    5. 不定式的被动形式
     当不定式的逻辑主语是动作的承受者时，不定式要用被动形式，包括它的一般式和完成式。例如：
        The snow was supposed to have blown off the mountain.
    6. 不定式的否定形式
     否定形式是在不定式的标志 to 前加 not 。例如：
    I decided not to ask him again.
    Please remember not to leave the lights on when are out.

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