这是固定词语搭配,这个你可以参照这个来看,以后常用背过了就自然了,不要钻牛角尖
“疑问词+不定式动词”其主要功用有下列五种：
⑴当主语,如：
● When to hold the meeting has not yet been decided.
● Where to live is a problem.
● How to cope with the rising cost of living becomes a daily discussion topic.
⑵当宾语,如：
● We must know what to say at a meeting.
● He could not tell whom to trust.
● Do you know how to play bridge?
⑶当补足语,如：
● The problem is where to find the financial aid.
● The question is who to elect.
⑷当名词同位语,如：
● Tom had no idea which book to read first.
● Do you have a rough impression how to do it?
⑸当宾语补足语,如：
● Jim is not sure whose to choose.
● Mary and John are not certain whether to get married or not.
适用于“疑问词+不定式动词”的动词包括：“know,see,decide,tell,ask,consider,discover,explain,forget,guess,hear,imagine,inquire,learn,remember,think,wonder,understand”等.
有点值得特别注意的是：当这结构当宾语时,它的作用等于名词分句,例如：
● I could not decide which dictonary to buy./ I could not decide which dictionary I should buy.
● Jack did not know where to find such a good teacher./ Jack did not know where he could find such a good teacher.
有些动词,如“ask,show,tell,advise,inform,teach”等,可以先有个宾语,然后才接着加上适当的“疑问词+不定式动词” 结构.例如：
● The chief technician showed the apprentice how to repair the machine.
● Have you told him where to get the application form?
