求if 和 as if在使用上的区别，求详细讲解一下。

1. CONJYou use
if in conditional sentences to introduce the circumstances in which an
event or situation might happen, might be happening, or might have happened.
(引导条件从句) 如果
例：She gets very upset if I exclude her from anything.
如果有什么事我将她排除在外，她就会非常难过。
例：You can go if you want.如果你想走就走吧。
2. CONJYou use
if in indirect questions where the answer is either "yes" or "no."
(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。
3. CONJYou use
if to suggest that something might be slightly different from what you
are stating in the main part of the sentence, for example, that there might be
slightly more or less of a particular quality. (表示与主句所述事实可能有少许不同) 即使
例：Sometimes that standard is quite difficult, if not impossible, to achieve.
有时那个标准，即使并非不可能，也是很难达到的。
4. CONJYou use
if, usually with "can," "could," "may," or "might," in a conversation
when you are politely trying to make a point, change the subject, or interrupt
another speaker. 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。
5. CONJYou use
if at or near the beginning of a clause when politely asking someone to
do something. (用于从句句首或靠近句首的位置，表示礼貌的请求) 是否[礼貌]
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？
6. PHRASEYou
use if not in front of a word or phrase to indicate that your statement
does not apply to that word or phrase, but to something closely related to it
that you also mention. 即便不
例：She understood his meaning, if not his words, and took his advice.
她即便不懂他的语言，也明白了他的意思，并且接受了他的建议。
7. CONJYou use if to introduce a subordinate clause in which you
admit a fact that you regard as less important than the statement in the main
clause. (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。
8. PHRASEYou
use if ever with past tenses when you are introducing a description of a
person or thing, to emphasize how appropriate it is. (与过去式连用，强调对人或事物的描述极为恰当) 如果有…的话[强调]
例：I became a distraught, worried mother, a useless role if ever there was one.
如果有个心烦意乱、忧心忡忡的母亲，一个完全无用的角色，那就是我。
9. PHRASEYou
use if only with past tenses to introduce what you think is a fairly good
reason for doing something, although you realize it may not be a very good one.
即使仅仅 (与过去式连用，引出能勉强说明做某事的原因)
例：She always writes me once a month, if only to scold me because I haven't answered her last letter yet.
她总是一个月写一封信给我，即使仅仅是为了责备我还没有回她的上一封信。
10. PHRASEYou
use if only to express a wish or desire, especially one that cannot be
fulfilled. (尤指不能实现的愿望) 要是…多好[情感]
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。
11. PHRASEYou use as if when you are making a judgment about something
that you see or notice. Your belief or impression might be correct, or it might
be wrong. (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。
12. PHRASEYou use as if to describe something or someone by comparing
them with another thing or person. (表示类比) 仿佛
例：He points two fingers at his head, as if he were holding a gun.
他用两个手指对着自己的脑袋，仿佛正拿着一把枪。
13. PHRASEYou use as if to emphasize that something is not true.
(强调某事不是真的) 好像…一样[口语]
例：Getting my work done! My God! As if it mattered.
把我的活儿干完！我的天哪！好像这很要紧似的。

as if 从句的作用
1．引导表语从句， 常与as,look, seem,feel,taste,smell,sound等系动词连用。
如：She looks as if she were ten years younger．
她看起来好像年轻了十岁。
2. 引导方式状语从句，表示“事情是怎么做的”或“一个人或事物是什么样的”。
如：
如：She loves the boy as if she were his mother．
她爱这男孩，就好像她是他的妈妈一样。
3.as if 还可用于省略句中，如果as if 引导的从句是“主语+系动词”结构，可省略主语和系动词，这样as if 后就只剩下名词、不定式、形容词（短语）、介词短语或分词。
如：He acts as if （he were） a fool．
他做事像个傻子。

as [强 æz; 弱əz]
adv.
1. [表示程度]同样地，相同地，一样地，相等地，达到…程度
2. [后接形容词或分词]被认为，被看作是，(从某种方式)与…对比时
3. [后接 directed, agreed, promised 等分词]照…一样,按照(规定的、同意的、答应的)方式
4. 诸如，例如
conj.
1. [表示比较]像…(一样)，达到…程度，以至于：
It flew straight as an arrow.

它直飞如箭。

You are as good as you think you are.

你像你自己认为的一样好。

2. [表示方式]以…的方式，按照；如同，如同…那样：
Do as you are told.

照吩咐去做。

Do as we do.

我们怎么干，你就怎么干。

3. [表示时间] 当…的时候，在…的同时，和…同时；当，正值：
She wept as she spoke.

她边说边哭。

Pay as you enter.

进来时付钱。

4. [表示原因]因为，由于，鉴于；既然：
As you object, we won't go.

由于你反对，我们不去了。

As you are leaving last, lock up.

你是最后一个离开，把门锁好。

5. [与so连用，表示结果或目的]：以至于，结果是：
The question is so obvious as to need no reply.

这问题很清楚，不必回答。

6. [表示让步，词序倒装]虽然，尽管，纵：
Tall as she was she couldn't reach the oranges.

她虽然长得这么高，仍够不着那些橙子。

Strange as it seems,it is so.

虽然看来很怪，但事实如此。

7. [口语][引导名词从句，在主从复合句的主句中]＝ that:
I don't know as I should.

我不知道我该这么做。

I don't know as I do.

我不知道我知道。

8. [与 so 连用，表示目的]为了，为使，以便：
Students should take notes so as to make revision easier.

为了便于复习，学生应该记笔记。

9. [常与 so 连用，表示比例或关系]正像；随着…：
As the wind blew harder,so the sea grew rougher.

风越刮越大，海浪也越来越汹涌。

10. 仿佛，好像( ＝ as if)：
It seems as she's never happy.

她似乎生活得一点也不愉快。

11. 根据；[与 go 连用]按照…的一般标准来说：
as far as it goes

就一般标准来说，就其本身来说

12. [口语][引导名词从句， ＝ that]
13. [方言]比；与其…(不如…)( ＝ than)
pron.
1. 如(情形或事实所表示)；这一点，这一事实，那一情况
2. [用于比较，前为such 或 the same，或与后跟形容词的 as 连用，引导定语从句，等于who，which或that]照此；如；像…样的人(或事物)；凡是…的人(或事物)
3. 也一样( ＝ and so)
4. [方言][引导定语从句,＝ who,whom,which 或 that]
prep.
1. 当作；作为；以…的资格，以…的身份：
He poses as a friend.

他摆出一副朋友的样子。

I do not think much of her as a musician.

她作为一名音乐家，我认为不怎么样。

2. 像，如同，跟…一样：
The risk is as nothing compared to the gain.

所冒风险与所得相比不足挂齿。

The thief disguised himself as a policeman.

那贼乔装打扮成警察。

3. [表示比较]像…(一样)：
as early as 1848

早在1848年

eyes as clear as crystals

水晶般明亮的眼睛

4. 例如：
a card game, as bridge

一种牌戏，例如桥牌

In some words in English the initial h is not sounded,as honour,hour,honesty,etc.

英语里有些词如 honour，hour，honesty等等的首字h不发音。

5. 为了…的目的：
I only said it as a joke.

我只是开玩笑说说而已。

6. 以…的形式：
We all start life as babies.

我们都是从婴儿开始人生的旅程的。

7. 由于…的理由[as being的省略形式]：
He was detained as suspects.

他因是嫌疑犯被拘留。

8. 起…的作用；(在剧中)扮演…的角色：
The illustration shows Mr.John as prince in Hamlet.

这个插图说明约翰先生在《哈姆雷特》一剧中扮演王子这个角色。

9. 当某人在…的时候：
As a child,he was rather naughty.

他小时候挺淘气。

as if
1. 好像，似乎，仿佛
2. [引导名词从句] ＝ that
3. [引导无定形句，表示惊讶、不信任、愤懑等强烈感情]好像…似的，倒是…似的

例句：
In August, long stalks with several clumps of trumpet-like flowers suddenly appear as if by magic.
在八月份，长长的茎上有一小簇像喇叭状的花，变魔术般的突然出现。

That is why this idiom is used to describe ideas that are not realistic, as if they are dreams or fantasies.
那就是为什么这个习语是用来描述不切实际的想法的，那些（想法）就好像做梦和幻想一样。

Sorghum is still used widely today in Africa. Now it looks as if it might be part of the original family recipe.
到了今天，高粱在非洲仍为人们广泛食用。目前看来，它可能早已是人们家传食物秘方的一部分