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。