that的用法：
1.that用作指示代词的意思是“那,那个”。可指前面提到的那件事,也可指在空间或时间上较另一事物远的事物或人,还可指要表明的或要提及的事物、想法等。
2.that用作关系代词时,指人或事物,同时又在从句中充当句子成分。that偶尔也可引导非限制性定语从句。
3.that还常用以代替who, whom, which,但that...which中的that用做关系代词的先行词。
4.that用作关系代词时在下列情况下可以省略:引导限制性定语从句在从句中用作宾语时; 用在there be结构前作主语时; 用作表语时; 在定语从句与先行词之间有插入语时; 在先行词way, reason, distance, place, office等作方式状语、原因状语或地点状语时。
5.that用作连词时可引导主语从句、宾语从句、表语从句、同位语从句和状语从句。
6.that还常引导间接引语或形容词后面的从句。
7.that在以it作形式主语的句子中或引导宾语从句、状语从句时常可省略。
8.that也可用于表示愿望或遗憾。
if的用法：
1.if作为从属连词,可引导条件状语从句、让步状语从句和名词从句。
2.if引导条件状语从句,从句可为真实条件从句,也可为虚拟条件从句。如为真实条件从句,谓语用陈述语气,表示可能性很大,作“假使”“如果”解,通常用一般现在时代替一般将来时; 如果if从句中用will,表示意愿。
3.if引导的条件从句中可以省略be和主语。
4.if引导的条件从句,有时后接only以加强其语气,表示说话人非常盼望某人提出的条件能够实现。
5.if可引导让步状语从句,作“虽然,即使”解,常和even连用。
6.if可引导名词性从句,作“是否”解,常放在ask, doubt, know, learn, see, wonder等动词后。一般用在口语中,而且不能用于句首。
7.if可以表示惊奇或愤怒。用于陈述语气的否定句时,意义则是肯定的。
8.If ever...在现代英语中,表示强调,用于加强主句的意思,往往含有否定意思。
9.When or if〔when, or if〕表示从句所说的也许不会实现。
10.if any的意思是“如果有的话”。
11.if only可表示对现时或未来的愿望,也可表示与过去事实相反的愿望,作“只要…但愿,要是…就好了”解。
12.What will happen if...?意思是“要是…怎么办”。常缩略成What if...?
13.If not后可加动词,通常动词省略,可用于现在时态或现在完成时态,表示“要不,不然”; 也可用于yes〔no〕疑问句之后,表示“许诺,告诫”。
(表示条件)如果 ,主句用将来时，if从句用现在时表示将来
He will come if you invite him.
如果你请他,他会来的。
2. (表示虚拟)假如,要是
1）时态：可以表示过去，现在和将来的情况。它的基本特点是时态退后。
a. 同现在事实相反的假设。
句型 ： 条件从句 主句
一般过去时 should( would) +动词原形
If they were here, they would help you.
b. 表示于过去事实相反的假设。
句型： 条件从句 主句
过去完成时 should(would) have+ 过去分词
If she had worked harder, she would have succeeded.
The rice would not have been burnt if you had been more careful.
If my lawyer had been here last Saturday, he would have prevented me from going.
If he had come yesterday, I should / would have told him about it.
含义：He did not come yesterday, so I did not tell him about it.
If he had not been ill and missed many classes, he would have made greater progress.
含义： He was ill and missed many lessons, so he did not make greater progress.
c. 表示对将来的假想
句型： 条件从句 主句
一般过去时 should+ 动词原形
were+ 不定式 would + 动词原形
should+ 动词原形
If you succeeded, everything would be all right.
If you should succeed, everything would be all right.
If you were to succeed, everything would be all right.
3. 是否 = whether 连接宾语从句
I wonder if she is ill.
不知她是否病了。
一、if作为连词，引导条件状语从句，表示“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（最基本的）。
eg. If it doesn’t rain, we will go to the picnic next Sunday. 如果天不下雨，下周日我们将去野餐。二、if还可以引导让步状语从句。在意义上相当于even if,even though或although。这时 if 意为“即使 是”，“虽然”。
eg. I’ll do it, even if it takes me a long time． 虽然会花费很长时间，我还是要做这事。 If he is little,he is strong.他年纪虽小,力气倒大
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：
eg. She asked if ／ whether I like it． 她问我是否喜欢它。 * 此时注意：1. or not放在whether之后时，只能用whether不用if。 2. 在强调任意选择时,用whether…or,此时不用if替换whether 3. 虽引导宾语从句，但为了强调宾语部分，也可把从句放在句首，此时只用whether 不用if 。 4. 从句作介词宾语时只用whether不用if。 5. 作discuss等动词的宾语时，用whether不用if。 6. 句子中有if引导的条件句，如再有表示“是否”的宾语从句，用whether不用if 7. 容易产生歧义时用whether不用if来表示“是否”。 基本上碰到选择 if 还是 whether 的问题时都选 whether，除了一条：如果宾语从句为否定句时，则 只用if不用whether。eg. I don’t know if it won't rain tomorrow..我不知道明天是不是会下雨 四、if引导时间状语从句，意为“当”或“无论何时”，不含有条件之义时，if从句中的时态与主句中的时 态相同。
eg. If you mix yellow and blue，you get green．你将黄色与蓝色混合，便会得到绿色。
五、if后接否定动词，用于感叹句中，表示沮丧、惊奇等。例如：
eg. Well，if I haven’t left my homework at home! 真倒霉，我把作业落在家里了!
六、用于虚拟语气中，if从句中用过去式，表示不太可能实现或提出作为考虑的假设条件。eg.
1. If I were a bird, I could fly.如果我是只鸟，我就会飞了。
2. If I asked him(if I were to ask him) for a loan，would he agree?如果我向他借款，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。eg.
1. If they had started earlier，they would have arrived in time．要是他们早些动身，他们便可及时到达了。
2. If they had not started at that time，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。
第三，if 条件句的虚拟有时可省略，然后将主语与所用的限定动词(were，should，had)倒置。eg.
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 post，I would do the same ． 如果我处于你的位置，我也会这样干的。
3. Had I(=if I had)known earlier，I wouldn't go．如果我早一点知道，我就会不会走了。
七、if与其它一些介词的连用。
as与if连用，意思是：“仿佛、好像”同样引导从句。eg. It isn’t as if he doesn’t know the rules．(ie． He does know the rulers．)他不像不懂规则的样子。 (即他懂得规则) （ie=namely=that is to say=in other words： “即，那就是说”）
另外as if还可以引导感叹句。例如：
As if I would allow it! (ie．I would certainly not allow it!) 好像我会答应那样! (我才不会答应呢!)
* if与only连用。表示“如果...就好了”。例如：
1. If only he arrives in time!他若能及时到达就好了!
2. If only she had known about it!（But she didn’t know.）她那时要是知道（但她不知道）这事就好了! * 此时注意区别 only if...：“只要...”，相当于“if” eg. Only if you word hard，you'll be successful one day.
“that”在英文中是一个使用频率很高的单词。它有四种词性，并且句法及语法功能纷繁复杂。如果不能熟练掌握其用法，很可能会形成英语学习的一种障碍，从而影响其学习兴趣和效率。现将that的用法总结归纳如下：
第一、that 用作形容词（后接复数名词时用those）。
它用来指已被提到的人或物；也可表对比，指两个中较远的那个。
What about that book you borrowed from me last month？
请注意，that 有时候在句子中具有喜欢或轻蔑等感情色彩。
that little son of his 他那个小宝贝儿子
That George！乔治那家伙！（含有轻蔑语气）
第二、that 用作代词。
1. that 用作指示代词（复数形式是those），其指代意义同形容词用法，同时它还可以用作定语从句中的先行词；还可为了避免重复，代替前述名词。
That is what he told me.
What is that (which) you have got in your hand?
The price of rice is higher than that of flour.
2. that 用作关系代词，引导限制性定语从句。先行词可人可物，用法相当于who或which。（但是在下列情况下多用that：先行词既有人又有物时；先行词有形容词最高级、序数词、不定代词、very，only等修饰时；先行词是不定代词时）
He talked about the teachers and schools that he had visited.
I think it one of the most wonderful films that the film company has ever produced.
She has little information that is useful for our research.
Is there anything that I can do for you?
请注意，that 在定语从句中作宾语时通常可省略。
The books (that) I sent you will help you in your studies.
第三、that 用作连词，引导名词性从句,状语从句和强调句。
1. that名词性从句。
①引导宾语从句。及物动词后的引导词that可省略。
I didn't expect (that) he could win the championship.
The teacher pointed out that Tom was not working hard enough.
② 引导主语从句。通常采用it作形式主语的句型。
That the earth goes around the sun is known to everyone.
（It is known to everyone that the earth goes around the sun.）
③引导表语从句。
The trouble is that we are short of money.
④引导同位语从句。
引导同位语从句的that和引导定语从句的that是不同的。前者只起语法作用，在从句中不作任何成分；而后者在定语从句中作主语、宾语或表语。举例说明：
The news that he resigned from office surprised us.
The idea that he holds is very common nowadays on campus.
2. that引导状语从句
①引导目的状语从句。
Bring it nearer that I may see it better.
②引导结果状语从句。
What have I done that he should be so angry with me?
③引导原因状语从句。
I am afraid that I will fail in the driving test.
④引导让步状语从句。意为“虽然、尽管”。
Difficult that/as the task was, they managed to accomplish it on time.
⑤引导条件状语从句。意为“假使、假设”。
Supposing that you were in my position, what would you do?
On condition that you were lost in the desert, you should ask for help as soon as possible.
3. 引导强调句。
It is Mrs. White that makes the decision in her family, not her meek little husband.
It is an ill wind that blows nobody good.
第四、that用作副词。
1. that用作普通副词。
I was that/so angry I could have hit him.
2. that用作关系副词。引导定语从句，可以代替when，where， why或 in which，常可省略。
I will never forget the evening (that) / when we went to the theatre.
The house (that)/where I used to live has been knocked down.
第五，与that 有关的常见重要短语。
1. in that，意为“既然、因为”。
Criticism and self-criticism is necessary in that it helps us correct our mistakes.
2. now that，意为“既然、由于”。
Now that they have taken matters into their hands, the pace of events has quickened.
3. see (to it) that，意为“注意、务必做到、保证”。
We will see to it that she gets home early.
See to it that you are not late again.
4. seeing that，意为“鉴于、由于”。
Seeing that it is 8 o'clock, we'll wait no longer.
Seeing that he was busy with his work, I didn't disturb him.
以上是that主要用法的概述，希望能对你有所帮助。当然，that还有其他的一些用法，需要我们在学习的过程中不断地积累和总结。希望我们一起加油！！