有一读者问： 在什么情况下可用“to be+现在分词”?可以说 “He was seen to be playing outside the house.”吗？
“to be+现在分词”是不定式的进行式，它的功能与限定动词的进行时态基本相似，所以许多英语语法著作都语焉不详，一掠而过。它的用法与不定式也大体相同。现分几类举例如下：
1. 不定式进行式可置于某些动词之后，用作宾语，如：
(1)I will not interrupt you any longer. You want to be reading .
(2)Do you wish to be again wandering and inquiring ?
(3)We did not expect to be staying in first-class hotels, because this is an economy tour.
(4)I'll need to be rising now.
(5)We resolved to be going the next day.
(6)I'd like to be dining at home now.
(7)He talks only because he loves to be talking .
(8)He longed to be going up the beach.
亦可用作真实宾语,形式宾语是 it，如：
(9)I think it rather unnecessary in you to be advising me.
2. 不定式进行式可置于“某些动词+名（代）词”之后，用作宾语补足语，如：
(10)While we supposed the old man to be reading the Bible to his wife, the pair of hoary reprobates have whisked up the chimney.
(11)If you don't think it too little, take that to be going on with. （it指一小笔钱）
3. 不定式进行式置于某些动词的被动式之后，用作主语补足语。这种动词有say, report, believe, think, find, know, understand, suppose等。但请注意不定式进行式不可用在 see, hear 等感知动词的被动式之后，故不可说“He was seen to be playing outside the house.”,而须把 to be 去掉。实例如：
(12)He was said often to be staying up late.
(13)Egypt and Israel are reported to be making preparations for the September summit in Washington.
(14)He is believed to be coming .
(15)Come, I am supposed to be calling you to lunch.
4. 不定式进行式可置于某些不及物动词之后，用作状语，如：
(16)She flushed to the roots of her hair to be explaining that to him and he smiled apologetically.
(17)I thought they were talking too much to be doing their work properly.
5. 不定式进行式可用于 have, have got, be going, seem, happen等动词之后（在这种结构中，be to, have to, be going to, seem to, happen to 等实际上已成为情态动词）。实例如：
(18)Doing one thing always reminded him that presently he would have to be doing another.
(19)You've got to be kidding .
(20)I'm going to be asking a lot of questions at the meeting this afternoon.
(21)He seemed always to be either washing his hands or wiping his boots.
(22)When he came in, I happened to be lying on the bed, reading.
6. 不定式进行式也常置于某些形容词之后，用作状语，如：
(23)He was returned very eager to be improving his own place.
(24)His fingers were ever straying as if impatient to be playing upon his pipe.
(25)He lay down with his eyes closed, glad to be absorbing the sun.
(26)You are much too young to be meeting young men.
(27)I am not likely to be seeing her.
(28)She is anxious to be going abroad.
有时也可置于“形容词+for+名（代）词”之后，与 for 短语构成复合结构（用作状语），如：
(29)It is rather late in the day for you to be taking precautions.
7. 不定式进行式可置于副词 enough 之后，用作状语，如：
(30)He was driving fast enough to be going for a doctor.
8. 不定式进行式可置于名词之后，用作定语，如：
(31)It's time to be going home, your mother will be anxious.
(32)You've no need to be fearing .
(33)He has no right to be offering presents to any men.
9. 不定式进行式也可用作主语，如：
(34)Just to be doing something again was a help.
也可以用作真实宾语（形式主语是it）,如：
(35)It's nice to be sitting here with you.
(36)It was pleasant to be reading and resting for change.
最后，请注意下列几点：
1. 不定式进行式在英语中用得较少（我们在此仅指“to be+现在分词”，不包括“情态动词+be+现在分词”）。
2. 不定式进行式常用以表将来，强调时间因素，往往表“即将”，试比较：
(37)He is believed to be coming.（即将来）
(38)He is believed to come. （to come 只表“会来”，不强调具体时间）
3. 不定式进行式和一般进行时态一样，常表一种持续性动作，也可以说是表一种状态。有一位外国学者对下面一句话中的 to be looking down 和一般不定式进行时做了比较：
(39)The soup was bad, and Miss Vereker left most of hers and contrived to be looking down at it very curiously every time Mrs. Dersingham glanced across the table at her.
他说这里的 to be looking down 的意思是“一直不停地往下看着（那汤）”。如用 to look down, 则表一种单纯的动作，其含义就变为：每当 Mrs.Dersingham 瞟 Miss Vereker 一眼,Miss Vereker 就往下看一看。
4. 和一般现在分词相比，不定式进行式强调事实，而现在分词则强调描绘性，试比较：
(40)He was found to be sleeping.（强调“在睡觉”）
(41)He was found sleeping. （强调“在睡着”）
