understand的過去式和過去分詞是什么

　　例句：

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

　　據(jù)我的經(jīng)驗看，真正理解這個問題的人很少。

　　I quite understand that you need some time alone.

　　我很理解你需要獨(dú)自靜一會。

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

　　人往往對自己不懂的東西感到恐懼。