buy的过去式和过去分词是？

　　例句：

　　The curtains were an impulse buy.

　　这窗帘是一时心血来潮买下的.。

　　We need to buy some new furniture.

　　我们需要买一些新家具。

　　Now is a good time to buy a house.

　　现在买房子正是时候。

　　They only buy the best.

　　他们只买最好的。

　　Try the shoes on before you buy them.

　　鞋子要先穿上试一试再买。

　　It may be necessary to buy a new one.

　　也许有必要买个新的了。