1. Those are all really important questions and very difficult to answer.
句式“be + 形容词 + 不定式”意为“很……做某事……”。如：
This kind of work is easy to complete. 这种工作很容易完成。
2. I’m not sure how to answer them.
不定式可以与疑问词 who, what, which, when, how, where 等连用，构成不定式短语，在句中作主语、表语、宾语和宾语补足语等。如：
Where to buy this kind of computer is a problem. 到哪里去买这种计算机是一个问题。
I really don’t know what to do. 我的确不知道该怎么办了。
3. The important thing is not to stop questioning.
这句话中的not to stop questioning 是动名词的否定形式，它由否定副词 not 加上动名词构成。在这句话中，动名词用在系词后作表语。如：
He is worried about not going to the concert. 他担心的是不能参加音乐会。
