how为连接副词，引导名词性从句，it后动词可为：一般现在时：动词单数形；一般过去时：动词过去式；还可接情态动词：I don't know how it happened.She will tell you how it can be used.
双语例句
1、Yeah, no, how could he do that?
是啊，不，他怎么可以那样呢?。
2、What? How could you do that?
什么？你怎么可以这样做？
3、How do I know what I should do, what not to do?
我怎么知道我应该做什么，不应该做什么呢？