(七)不定式的时态。动词不定式一般有三种时态形式，即一般式、进行式和完成式。

(1) 一般式(to+动词原形)：表示与主句谓语动词动作同时(或几乎同时)发生或在主句谓语动词之后发生的事情。例如：

We’re happy to have you on our side. 有你在我们这边我们很高兴。

I saw him go out. 我看见他出去了。( 不定式go out 与 see同时发生。)

They invited me to have dinner with them.  他们邀请我和他们一起吃晚饭。

I hope to see you again. 我希望再见到你(to see发生在hope之后)

(2) 进行式(to be+现在分词)：表示主句谓语的动作发生时，不定式动作正在进行。例如：

She’s said /believed to be living nearby. 据说/据信她就住在附近。

When he came in, I happened to be sleeping in bed. 他进来时，我碰巧正在床上睡觉。

They seem/appear to be enjoying themselves. 看上去他们似乎很快活。

He pretended to be doing his homework when he heard his father’s sound. 他听见父亲的声音，假装正在做作业。

The president was reported to be visiting the hospital. 据报道总统正在访问那家医院。

(3)完成式(to have+过去分词)：表示在主句谓语的动作之前发生的事情。

如：I’m glad to have met your parents here. 我很高兴在这儿见到了你父母。(已见过)

I’m sorry to have kept him waiting for me so long. 我很抱歉让他等了我这样久。(已经等过了)

(八)不定式的被动语态。当不定式的逻辑主语是这个不定式所表示的动作的承受者时，不定式一般要用被动形式。

不定式的被动形式有两种：一般式 to be+过去分词；完成式 to have been+过去分词。例如：

You’re lucky to have been accepted . 你很幸运已经被接受了。

These criminals are to be hanged .  这些罪犯是将被绞死的。

We’re glad to have been invited.   受到邀请我们很高兴。

What’s to be done next? 下一步做什么?

It is an honor for me to be asked to speak here.   我很荣幸应邀在这里讲话。

为了避免重复，不定式可省略，但to有时保留，有时不保留。例如：

You will make it if you try (to). 如果你努力，你会成功的。

George says he is going to leave Shanghai, but I don’t think he really wants to. George说要离开上海，但是我不认为他真的愿意走。
Some of them retired, and others were ready to. 一些人退休了，还有的准备退。

He always speaks faster than he needs to. 他说话总是没必要的快。

I’d like to do it now, but I haven’t got the time (to).  我倒是想现在干，但没时间。

“Would you go there with me?” “I’m glad to.” “你愿意和我一起去吗?”“愿意。”

“Did you pass the exam?” “No, I tried (to), but I failed.” “你考试及格了吗?”“没有，我努力了，但没成功。

“Do you want to sing them an English song?” “I prefer not to.” “你愿意为他们唱首英文歌吗?”“我不想唱。”

You’d better sing an English song if they ask you (to) again.如果他们再要求你，你最后唱一首英文歌。

后面常省略不定式而保留to的结构，还有：
used to   常常 be going to 打算 mean to  打算 ought to   应该 plan to 计划 want to  想要

(九)两个不定式由and, or, except, but, than 连接时，第二个可省略to，尤其是两个不定式紧密相连时。例如：

I intend to call on him and discuss this question again. 我打算去拜访他并和他谈谈这个问题。

I’d like to lie down and go to sleep. 我想躺下睡觉。

You’re free to talk or laugh here. 在这里你可以随便说笑。

I’m anxious to go and ask him about it. 我急于去问问他这个事。

Do you want to have lunch now or wait till later?   你想现在吃午饭还是再等会儿?

We had nothing to do except (to) look at the posters outside the cinema. 我们无事可做，只有看看电影院外面的招贴。

He had nothing to do except talk nonsense.   除了胡说八道，他没别的事干。

I would rather die than be insulted. 我宁死也不受侮辱。