疑问代词what, which, whom和疑问副词where, when, how后可以跟不定式,构成短语,如:what to do (做什么),where to go (去哪里)等。这类短语常用作宾语,可以转换为一个宾语从句。但要注意,这种结构转换为宾语从句时,要补加相应的主语,而且从句中谓语动词的时态常用一般将来时shall或will (过去将来时用would或should),或用情态动词can (过去时用could)。
I don't know what to do. 我不知道做什么 。
I don't know what I shall do .
Could you tell me where to buy the book? 你能告诉我在哪里买那本书吗 ?
Could you tell me where I can buy the book ? (could是更委婉的说法,不是过去时 )
He didn't tell us when to leave . 他没有告诉我们什么时候动身 。
He didn't tell us when we should leave .
He didn't tell us when we could leave . (主句是过去时,所以从句用过去有关时态 )
I don't know whom to go with . 我不知道同谁一起去 。
I don't know whom I shall go with . (with不可省略,因为只能说go with sb., 不能说go sb., go是不及物动词 )
Do you know which room to live in ? 你知道住哪个房间吗 ?
Do you know which room we shall live in ? (从句的主语应该用we。另外,in不可省略,因为只能说live in a room, 不能说live a room, 这里的live是不及物动词 )
She showed me how to plant trees . 她向我示范怎样栽树 。
She showed me how I could plant trees . (从句的主语应该用I )