understand的过去式和过去分词是什么

例句：

In my experience, very few people really understand the problem.

据我的经验看，真正理解这个问题的`人很少。

I quite understand that you need some time alone.

我很理解你需要独自静一会。

People are often afraid of things they don't understand.

人往往对自己不懂的东西感到恐惧。