if
（表条件）如果；即使；是否；（表假设）假如
(引导条件
从句) 如果
例：You can go if you want.
如果你想走就走吧。
(用于间接是非问句中) 是否
例：He asked if I had left with you, and I said no.
他问我是不是和你一起离开的，我说不是。
常与can、could、may、might等
情态动词连用，表示婉转客气
例：If I could just make another small point about the weightlifters in the Olympics.
请允许我就奥运会的举重运动员们再稍说一句。
(用于从句句首或靠近句首的位置，表示礼貌的请求) 是否
例：I wonder if you'd be kind enough to give us some information, please?
不知能否请您向我们提供些信息？
(引导从句，提出次要的事实) 即使
例：If there was any disappointment it was probably temporary.
即使有任何遗憾，可能也是暂时的。
(尤指不能实现的愿望) 要是…多好
例：If only you had told me that some time ago.
要是你之前告诉我那件事该多好。
(表示判断) 好像
例：It looked
as if she had forgotten how to breathe.
她看上去好像是忘了如何呼吸似的。
[表示条件或假设]如果；假如，倘若，要是：
例：If I knew, I would say.
如果我知道，我就说了。
例：If I were you, I would have failed.
如果我是你，我可能会失败的。
尽管；虽然；即使：
例：If he is little, he is strong.
他尽管年纪小，力气倒蛮大的。
例：If I am wrong, you are not right.
即使我错了，你也不对。