buy的過去式和過去分詞是？

　　例句：

　　The curtains were an impulse buy.

　　這窗簾是一時心血來潮買下的。

　　We need to buy some new furniture.

　　我們需要買一些新家具。

　　Now is a good time to buy a house.

　　現在買房子正是時候。

　　They only buy the best.

　　他們只買最好的。

　　Try the shoes on before you buy them.

　　鞋子要先穿上試一試再買。

　　It may be necessary to buy a new one.

　　也許有必要買個新的了。