首先我告诉你,这不是一个从句.句子的主语是what to do这是一个疑问词加不定式结构,不是句子,所以整个句子也不算从句.What I should do is a problem.这才是个主语从句.
参看：
英语有个很有用的结构,就是：疑问词+不定式动词.
疑问词是疑问代词“who,whom,what,which,whose”和疑问副词“when,where,how,why ”.此外,连接词“whether”也适用.
“疑问词+不定式动词”结构有何功用呢?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
