if是如果 providing that和provided that都是作为前提条件能举个例子吗，比如说啥时候只能用if，谢谢！If my best friend was a boy，he would love me. 虚拟语态 说明mu best friend 不是男孩 如果是provided that my best friend is a boy，he will love me.没有具体说我的哪个best friend，只是说假如我有一个男性朋友