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if and only if和only if的区别
only if：只有、除非,意思相当于unless,但用于肯定句,引导条件状语从句：I will go only if you go with me.除非你跟我一起走,否则我就不走（只有你跟我一起走我才走）.
比较：unless,用于否定句：I won't go unless you go with me.
If only：只要,引导条件状语从句：If only it clears up,we'll go.只要天一放晴,我们就走.
要是……就好了,引导独立句：If only you had won the game!你要是赢了该多好啊.
Only if 用于句首，用倒装语序，Only if a teacher has given permission is a student allowed to enter this room. 只有得到老师的允许，学生才能进入这间教室。
if only： 但愿 表示对现在和将来的愿望，或与过去事实相反的愿望。 If only it would stop raining 真希望雨能停。
一、if only的用法。
if only后面跟的是虚拟语气的句子，表示与事实相反的假设。
If only I had more money, I could buy some new clothes. 要是我有更多的钱，我可以买些新衣服。
If only he had arrived in time. 要是他能及时赶到就好了。
If only I had known it, I wouldn't have troubled him. 要是我知道，我就不会麻烦他了。
If only that photograph weren't missing. 要是那张照片没丢失该多好。
二、only if的用法。
only if后面跟条件状语从句，用陈述语气，后面用部分倒装句，be动词、助动词和情态动词放在主语之前，表示一种充分非必要条件，意思是只要。
Only if you passed the entrance exam and became a college student, will I accept you as a friend.
只要你通过高考，成为大学生，我就会认你为朋友。
Only if we persist in carrying out the open door policy, will we achieve greater success in every field.
只要我们坚持改革开放，我们就能在各个方面取得更大的成就。
onlyif与ifonly区别?
only if与ifonly的区别：意思不同、用法不同、侧重点不同
一、意思不同
1.only if意思：只有当(只是在 ... 的时候)
2.ifonly意思：仅限于
二、用法不同
1.only if用法：可修饰单数名词,也可修饰复数名词。前面常加定冠词the或形容词性物主代词。
例句：
The only time to dress up is when your jeans are dirty.
只有当你的牛仔裤变脏的时候你才会去打扮一下。
2.ifonly用法：用在名词短语之前,尤其是用在以数量词或数字起始的名词短语之前,起强调作用。
例句：
Ifonly I were as rich as you!
我要是能像你一样富有该多好啊！
三、侧重点不同
1.only if侧重点：用于一般现在时。
2.ifonly侧重点：用于一般现在时或一般将来时。
if and only if和only if的区别
only if (用于句首,后接从句主谓倒装)只要;只有conj.只有当(只是在 ...的时候)
例句:Only if I get a job will I have enough money to go to school
这两个完全不同，那需要区别？
unless===if not
ONLY if===only when
Only if意为只有当 ......
例：Only if you study hard can you get good grades.一般后面的主句用倒装，中间不隔开。
unless是除非的意思
例:I won't go to the party unless invited.
.
我只有找到了工作才有足够的钱上学.I can buy the house only if ...
if和unless的用法区别条件状语从句
这两个词放在句首或句尾在用法上是一样的,只是族在句首,就是两个句子,放在句中,就是一个句子,也就是说是用if或unless将两个句子连接成一个句子.
例:If it were not for your help,I would not have succeeded.要不是你的帮忙,我是不会成功的.
= I would not have succeeded if it were not for your help.
unless = if.not用来引导否定的条件句,也就是说用来代替否定的if从句.
例:If Max doesn't come on time,we will leave without him.如果Max没准时来,我们就不等他先走.
=Unless Max comes on time,we will leave without him.
not+unless=only...if 表示"如果...的话"
例:Venus won't come unless Nancy does.Venus不会来,除非Nancy有来.
=Venus will not oonly come if Nancy does.
注意,这里的only只是用来强调if ,与 if only(要是...就好了) 意思不同喔~
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