unless if once用法的区别

首先意思上就不同 unless 除非 if 如果 once 一旦
unless
1、 if not 除非
we won't get there on time unless we leave earlier.
除非我们早点离开,否则不能准时到.
2、except when 除...之外,除...的情况之外
The directors have a meeting every Friday,unless there is nothing to discuss.
主任们每星期五开会,除了没事讨论以外.
if 的虚拟用法
if引导的虚拟条件句中如果有should,were,had时,可以把should,were,had提前,if省略,句子倒装.
拟语气在as if/as though、even if/even though等引导的表语从句或状语从句中,
如果从句表示的动作发生在过去,用过去完成时；
指现在状况,则用一般过去时；
指将来状况则用过去将来时.
如果从句表示的动作发生在过去,用过去完成时；
指现在状况,则用一般过去时；
指将来状况则用过去将来时.
如：He did it as if he were an expert.
Even if she were here,she could not solve the problem
if
连接词 conj.
1.(表示条件)如果
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
If I were you,I would never do that.
如果我是你,我决不做那件事.
3.(表示让步)即使
We'll go even if it rains.
即使下雨我们也要去.
4.是否
I wonder if she is ill.
不知她是否病了.
5.(表示因果关系)每一次...的时候(总是会)
If I feel any doubt,I inquire.
我一感到有疑问就随时询问.
名词 n.
1.条件;设想[C]
There are too many ifs in his offer.
他的提议中条件太多了.
IF=i.f.
if.
缩写词 abbr.
1.=information feedback 信息回馈
2.=interferon
3.=intermediate frequency 中频
i.f.
缩写词 abbr.
1.=intermediate frequency 中频
if从句的用法：
一,引导条件状语从句,“如果,假设”的意思,
If you don't go there,I won't,either.如果你不去,我也不去.
二,引导宾语从句,“是否”的意思,如：
He asked if I could help him他问我是否帮下他.
once：
做副词
1.一次,一回.
eg：I saw him once and never again.我见过他一回,以后再也没见到过.
Once a week ,she wrote to her mother.她一星期一次给她妈妈写信.
2：曾经,一度
eg：She was oce my best friend.她曾是我最好的朋友.
3：用于条件句、否定句等,一旦
eg：If you once forget it,you won't come up with it again.你一旦忘记它,就不会再想起来.
4：用于否定句,一次也,完全
eg：I haven't seen him here once.我自爱这里一次也没见到过他.
I couldn't once get his meaning.我完全不能理解他的意思.
5：乘以1
eg：Once eight is eight.一乘八得八.
6：亲属关系上隔一代
eg：a cousin once removed 同（外）曾祖的堂（或表）兄弟（姐妹）
做名词
一次,一回
eg：Do it just this once .就做这一次.
做连词
一旦.就.
eg：Once you have taken the examination ,you will be able to relax.
你一旦考过,就可以轻松了.
做形容词
从前的,一度的
eg：the once capital of the nation 那个国家的旧都.