知識點：動詞need,require,want作“需要”解,其后跟動詞作它的賓語時,若表示的含義是被動的,必須用動名詞,或不定式的被動式。

例如：
The window needs(requires, wants)cleaning(to be cleaned).
在短語devote to, look forward to, pay attention to, stick to, be used to, object to,thank you for, excuse me for 等后的動詞也必須用動名詞形式：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,強調寫的過程,故應用現在分詞一般被動式)
Having been deserted by his guide, he couldn't find his way through the jungle.(為了強調已完成的動作)
Asked to stay, I couldn't very well refuse.
這里 asked 可能意味著 having been asked, 也可能意味著when/since I was asked, 但用了 having been asked 就不會有歧義。
下面句中過去分詞表示的時間與謂語動詞所表示的時間相同，所以不能代之以強調先于謂語動詞的現在分詞完成被動式。例如：
Covered with confusion, I left the room.我很窘地離開了房間。
United, we stand; divided, we fall. 團結則存，分裂則亡。
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?
但在正式文體中,用ought we not形式。例如:
We ought to go, ought we not?或We ought to go ,should we not?
動詞need,require,want后跟動詞作賓語時若表被動則必用動名詞,或不定式