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，他将会帮助你。
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 p oor，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。
四、if引导时间状语从句，当if做“当”或“无论何时”解而不含有条件之义时，if从句中的时态与主句中的时态相同。例如：
1. If youmix yellow and blue，you get green．你将黄色与蓝色混合，便会得到绿色。
2. If she wants the servant，she rings the bell．每当她需要仆人时，她便按铃。
五、if后接否定动词，用于感叹句中，表示沮丧、惊奇等。例如：
I. Well，if I haven’t left my false teeth at home!真倒霉，我把假牙丢在家里了!
2. And if he didn’t try to knock me down!(What do you think he did!He tried knock me down!)
六、用于虚拟语气中，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。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?
七、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)。 1.
If they had startedearlier，they would have arrivedin time．
If they had not started when they did，they would not be here now．
第三，在文学体栽中，if有时可省略，然后将主语与所用的限定动词(尤其是were，had，should)倒置。例如：
1. Should it (=if it should)be necessary，I will go. 倘若有必要，我会去的。
2. Were I(=if I were) in your place，I would do the same ．
3. Had I(=if I had)known earlier，I would come．如果我早一点知道，我就会来。
八、if与其它一些介词的连用。
（1）as与if连用，它的意思是：“仿佛、好像”同样引导从句。例如：
1. It isn’t as if we are rich．(We are not rich．)我们不像富有的样子。(即我们不富有)
2. It isn’t as if he doesn’t know the ruler．(i．e．He does know the rulers．)
（2）as if还可以引导感叹句。例如：
As if I would allow it!(i．e．I would certainly not allow it!)
（3）if与only连用。常表示愿望或一个未实现的条件，尤其用于感叹句中。例如：
1. If only he arrives in time!他若能及时到达就好了!
2. If only she would marry me!但愿她能嫁给我！
3. If only she had known about it!（But she didn’t know.）
1.if 可引导状语从句----“如果...,假使...”[就现在,过去,未来可能实现之事加以推测时]
If you are tired, we will go straight home 如果你疲倦了,我们就直接回家
If you have finished reading this book, please return it to me
Do you mind if I open the window? 如果我打开窗子,你介意吗?
I shall tell him if he comes 如果他来了,我会告诉他
2. 用于虚拟语气中
If you knew how I suffered, you would pity me
If I were you, I would help him 如果我是你,我会帮助他
If I had known, I wouldn't have done it 假使我知道,我就不会做那件事
If he had followed my advice, he wouldn't be in such difficulty now
If it should rain tomorrow, I shall not [shan't, won't] come
3.if可引导宾语从句,"是不是..."基本等同于whether
Ask him if it is true 问他那是不是真的
I wonder if he is in the school 我不知道他是否在学校
IF引导的条件状语从句，后半句即主句为将来时，从句则应为现在时。
例如：If he walks to school at 6:50am ,he will be late for school
IF有时候也引导虚拟语气
例如：If I were you, I would go with him.
since用法：
1．用作介词，意思是“从……以来，自从”，常与完成时连用
例如：I haven’t written home since Christmas.
2．用作副词，意思是“从那时以后(一直到现在)”，ever since从那以后一直(更加强调since 的用法)。
His first book was a great success and he has been famous as a writer ever since.
3．用作连词
4．引导原因状语从句，相当于now that，意思是“因为…，既然…，鉴于…”。
Since the rain has stopped，let’s go for a walk.
since表示原因时，语气比because弱，表示的原因是对方已知的，或是句中不很重要的部分，是附带的原因，因此引导的从句不是信息的焦点，不能回答why的提问，也不能用在强调句型中作为被强调的内容。since引导的从句往往放在主句之前。
5. 引导时间状语从句，此用法较复杂。
(1) 若since引导的状语从句的谓语动词是终止性动词(如go，come，leave，start，
begin等)的过去时，则从句表示的时间是“从该动作开始的那一时刻起”。这是最常见的
一种用法。如：
He has studied very hard since he came to our school.
(2) 若since引导的状语从句的谓语动词是持续性动词或表示状态的静态动词(如
live，stay，study，learn，smoke，be等)的过去时，则从句表示的时间是“从该
动作或状态的完成或结束时算起”。如：
I haven’t eaten snakes since I was a student at school.
(3) 若since引导的状语从句的谓语为持续性动词或静态动词的现在完成时，则
表示动作和状态延续到现在(说话时刻)，其表示的时间的起点应从动作发生之时算起。
如：
He has written to me frequently since I have been ill.
(4) It is + 段时间+ since... / It has been + 段时间+ since...表示“从……起已有
多长时间了”。两句型意思相同，前者是规范说法，后者是口语说法。
