知识点：动词need,require,want作“需要”解,其后跟动词作它的宾语时,若表示的含义是被动的,必须用动名词,或不定式的被动式。
例如：
The window needs(requires, wants)cleaning(to be cleaned).
在短语devote to, look forward to, pay attention to, stick to, be used to, object to,thank you for, excuse me for 等后的动词也必须用动名词形式：I look forward to hearing from you soon.
Badly polluted, the water cannot be drunk.(原因)
Being written in haste, the composition is full of mistakes.
(原因,强调写的过程,故应用现在分词一般被动式)
Having been deserted by his guide, he couldn't find his way through the jungle.(为了强调已完成的动作)
Asked to stay, I couldn't very well refuse.
这里 asked 可能意味着 having been asked, 也可能意味着when/since I was asked, 但用了 having been asked 就不会有歧义。
下面句中过去分词表示的时间与谓语动词所表示的时间相同，所以不能代之以强调先于谓语动词的现在分词完成被动式。例如：
Covered with confusion, I left the room.我很窘地离开了房间。
United, we stand; divided, we fall. 团结则存，分裂则亡。
He used to live in London, use(d)n't he /didn't he?
There used to be a cinema here before the war, use(d)n't there /didn't there?
Such things ought not to be allowed, ought they?
He ought to be punished, oughtn't he?
但在正式文体中,用ought we not形式。例如:
We ought to go, ought we not?或We ought to go ,should we not?
动词need,require,want后跟动词作宾语时若表被动则必用动名词,或不定式
