forget的过去式和现在分词怎么写

　　I forget how much they paid for it.

　　我忘了他们用多少钱买的这东西。

　　Let's forget our differences and be friends.

　　咱们别把分歧放在心上，做个朋友吧。

　　I completely forgot her birthday, what with one thing and another.

　　因为忙得不可开交，我把她的生日忘得一干二净。