1． 作宾语：在advice, ask, consider, decide, discover, discuss, explain, find out, forget,
　guess, imagine, know, learn, observe, remember, see, teach, tell, think, understand,
　wonder等动词后常用疑问词和带to的不定式作宾语，如：
　　I didn't know whether to go or not.
　　He asked me how to solve the problem.
　　I'll tell you what to do next tomorrow.

2) 做主语或表语，如：
　　When to do the experiment has not been decided yet.
　　The problem is how to maintain the quality.