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動(dòng)詞know后的不定式符號(hào)to的省略問題

當(dāng)動(dòng)詞know的后面跟有不定式作賓語補(bǔ)足語時(shí)（即用于“know+賓語+不定式”），該不定式有時(shí)帶to，有時(shí)不帶to，具體用法如下：

(1) 若know為一般現(xiàn)在時(shí)，則其后用作賓語補(bǔ)足語的不定式不能省略to（此時(shí)的不定式通常為to be型）。如：

I know him to be ill. 我知道他病了。

We know her to be honest. 我們知道她是誠實(shí)的。

(2) 若know為一般過去時(shí)或現(xiàn)在完成時(shí)，則其后用作賓語補(bǔ)足語的不定式可以省略to，也可以保留to（此時(shí)的不定式可以是to be型，也可以是to do型）。如：

I never knew him (to) do such a thing. 我從未聽說他會(huì)干那種事。

I’ve never known her (to) be late before. 我以前從不知道她遲到過。

I’ve never known it (to) snow in July before. 我從未聽說過有七月下雪的事。

I have never known a horse (to) eat fish. 我沒聽過馬會(huì)吃魚。

但是，當(dāng)用作賓語補(bǔ)足語的不定式為完成式時(shí)，其中的to不能省略。如：

They knew the man to have been a thief. 他們發(fā)現(xiàn)此人曾是個(gè)小偷。

I have never known her to have failed. 我從來沒發(fā)現(xiàn)她失敗過。

(3) 當(dāng)動(dòng)詞know本身用于被動(dòng)語態(tài)時(shí)，其后的不定式不能省略to。如：

He has never been known to tell a lie. 從未有人聽說過他說謊。

Smoking is known to increase a person’s risk of developing lung cancer. 據(jù)說吸煙會(huì)增加一個(gè)人患肺癌的風(fēng)險(xiǎn)。

People have been known to drive 500 miles just to visit the shop. 聽說有人駕車500英里只是為了去這家商店。