不定式是动词的非谓语形式，但它还保留着动词谓语形式的一些特征，具有时态（一般式、完成式和进行式）和语态（主动和被动）变化。
1．现在时态——现在时态所表示的动作跟谓语动词所表示的动作同时发生或在它之后发生。例如：
The scientist tries to apply newer way to production.【同时发生】科学家想把较新的方法应用到生产中去。
Who heard him say that? 【在后发生】谁听见了他说那种话？
2．完成时态——完成时态表示的动作在谓语动词的行为之前已经发生或完成。例如：
They are much pleased to have designed such an electronic computer．他们因为设计了这样一个电子计算机而感到很高兴。
The energy of coal is found to have come from the sun．人们发现煤的能量来自太阳。
I'm sorry to have troubled you so much．我很抱歉，麻烦了你这么多。
在助动词be和表示“意图”、“希望”、“需要”等意义的动词的过去式之后用不定式的完成时；或这类动词的过去完成时+不定式的一般式，均可表示“过去想做而未做的事情”或“行为没有实现”。这类动词有desire, expect, hope, intend, mean, plan，suppose, think, want, wish等。例如：
He was to have left home at nine o'clock．他本该九点钟离开家。
We planned to have started the work at the beginning of the month.我们原来计划月初开始这工作。
We wished to have solved the problem graphically．我们原想用图解法解算这道习题。
I had hoped to catch the 8:30 bus, but found it was gone．我原来希望赶上8点30分的车，但发现车已开走了。
I had meant to tell you about it earlier, but I could not come．我本想早点把这事告诉你，但我来不了。
用在would like, had better, should, ought, be supposed等之后，也可表示事情未曾发生过或愿望没有实现。例如：
She should have told him he could shower in the house if he wanted.她原该告诉他如果需要，可以用房子里的莲蓬头洗澡。
The letter should have been ten pages long．这封信本该有10页长。
You oughtn't to have spoken to them in that way．你本不该这样对他们说话。
在情态动词must, may, might, could或can之后用不定式完成时，说明对过去发生的事情或某种情况表示怀疑或推测。例如：
You must have used the wrong data yesterday．昨天你一定用了错误的数据了。
And he must have died alone, that's why he had the cameras sent to her．他死时一定是孤身一人，所以他才让人把相机寄给她。
He may have done the work very well．他也许已经把工作做得很出色吧。
Could she have forgotten to come？她可能忘记来了吗？
3．进行时态——进行时态表示的行为在谓语动词行为发生时正在进行。例如：
They were said to be making a new experiment．据说他们正在做一个新的实验。
The new satellite is reported to be flying around the earth．据报道新卫星正在绕地球飞行。
I don't like you to be always coming late for work．我不喜欢你上班总是迟到。
The weather seems to be improving．天气似乎正在好转。
情态动词must, may等之后接不定式的进行时表示对现在正在进行的事或状况的猜测或估计。例如：
Philip thought Rose must be looking forward to his return．菲利普想，露丝准是盼望他回来。
He may be reading the book．他也许正在读这本书。
4．完成进行时态——说明行为在谓语动词表示的行为之前开始，并延续到那个时候可能仍在进行。例如：
She was said to have been living in Guangzhou for ten years．据说她在广州一直住了十年。
He seems to have been writing an article on applied physics．他似乎一直在写一篇关于应用物理的论文。
This project is known to have been going on for three months．大家知道这个工程已经进行了三个月。
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