1. 一般来说，当不定式的逻辑主语是动作的执行者时，不定式用主动语态，是承受者时用被动语态。如：
Tom wants to help the college student. (逻辑主语是Tom)
He wants me to learn Russian. ( 逻辑主语是me)
The house to be built next year is a clinic. (逻辑主语是house)
2. have(有）后跟不定式作定语时，不定式用主动语态。如：
I have something to buy in town.
但是，若不定式的动作的执行者不是句子主语而是他人时，可用被动语态。如：
I am going to town. Do you have anything to be bought (by me)?
3. 形容词easy,difficult, good, fit,hard等后跟不定式时，往往用主动语态。如：
The text is easy to recite.
I find the problem hard to solve.
4. ”There be+主语+不定式（作定语）”时，不定式可用主动或被动语态。如：
There are lots of things to do/to be done.
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