1
[表示条件或假设]如果；假如,倘若,要是：
If I knew,I would say.
如果我知道,我就说了.
If I were you,I would have failed.
如果我是你,我可能会失败的.
2.
尽管；虽然；即使：
If he is little,he is strong.
他尽管年纪小,力气倒蛮大的.
If I am wrong,you are not right.
即使我错了,你也不对.
3.
是否：
I don't know if he is at home.
问他是否在家.
Ask him if he's at home.
我不知道他是否在家.