what to do的语法分析这算一个省略句吗?想问一下这里的主语在哪,我知道what做的是do的宾语,但从主谓宾角度分析的话,这句话应该怎么分析?

间接从属疑问常用句行 上面说的很清楚了
如I l know what to do. = I know what I must do.
这类常用的有 whom/where/how/which

一个句子中有一部分动词后可以接
比如：I don't know what to do.
(我不知道该做什么)

what to do并不是一个句子,而只是一个名词性成分,一般称为 “疑问词 + 不定式” 结构.
所以也就不能用句子的语法来分析该结构了.
不过如果放在具体句中时,可以换写成相应的名词性从句.
如：I don't know what to do.
= I don't know what I should do.
如果疑问词为代词时,则要求该疑问词必须做不定式中某个动词或介词的宾语.
再如：I don't know whom to send this letter to.
= I don't know whom I should send this letter to.
我不知道该把这封信送给谁.
此例句中,whom充当介词to的宾语,即：
whom to send this letter to
= to send this letter to whom

“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?