if做连词,主要有两个意思,“是否”,引导宾语从句；“如果”,引导状语从句：
一、if作为连词,引导条件状语从句,它表示的意思是“假如”“如果”等.在复合句中如果主句用将 来时,则if引导的状语从句用一般现在时.例如：
1.If itdoesn’train,we will go to the park next Sunday.如果天不下雨,下周星期天我们将去公园.
2.If you ask him,he will help you．如果你求他,
if从句中用过去式,表示不可能实现,大概不会实现或提出作为考虑的.假定条件.此时需要用虚拟语气,例如：
1.If you were a bird,you could fly.假使你是只鸟,你便会飞了.
2.If I asked him(if I were to ask him) for a loan,would he agree?如果我向他借贷,他会答应吗?
另外,if从句中用过去完成式,表示过去未实现的条件(例如由于不可能实现或某人之未能实行).例如：
1.If they had startedearlier,they would have arrivedin time．要是他们早些动身,他们便可及时到达了.
2.If they had not started when they did,they would not be here now．如果他们那时不动身,现在他们就不会在此地了.
二、if还可以引导让步状语从句.这时if当作“即使是”,“虽说”解.例如：
1.If she’s poor,at least she’s honest． 虽说她很穷,但至少她还是诚实的.
2.If I am wrong,you are wrong,too． 即使说我错了,那么你也不对.
3.I’ll do it,even if it takes me all the afternoon． 虽然会花费我一下午的时间,我还是要做这事.
三、if作为连词还可以引导宾语从句.引导宾语从句时和whether意思相同.例如：
1.Lily asked if ／ whether she liked it.莉莉问她是否喜欢它.
2.She asked if ／ whether they had a cotton one． 她问是否他们有一件棉织的.