fold的过去式和过去分词和现在分词是什么

　　例句：

　　You'd better fold it carefully.

　　你最好仔细地折叠好它。

　　I fold up the chairs.

　　我把椅子折起来。

　　The cost of a loaf of bread has increased five-fold.

　　一块面包的价格已经涨了5倍。