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.
除非我們?cè)琰c(diǎn)離開(kāi),否則不能準(zhǔn)時(shí)到.
2、except when 除...之外,除...的情況之外
The directors have a meeting every Friday,unless there is nothing to discuss.
主任們每星期五開(kāi)會(huì),除了沒(méi)事討論以外.
if 的虛擬用法
if引導(dǎo)的虛擬條件句中如果有should,were,had時(shí),可以把should,were,had提前,if省略,句子倒裝.
擬語(yǔ)氣在as if/as though、even if/even though等引導(dǎo)的表語(yǔ)從句或狀語(yǔ)從句中,
如果從句表示的動(dòng)作發(fā)生在過(guò)去,用過(guò)去完成時(shí)；
指現(xiàn)在狀況,則用一般過(guò)去時(shí)；
指將來(lái)狀況則用過(guò)去將來(lái)時(shí).
如果從句表示的動(dòng)作發(fā)生在過(guò)去,用過(guò)去完成時(shí)；
指現(xiàn)在狀況,則用一般過(guò)去時(shí)；
指將來(lái)狀況則用過(guò)去將來(lái)時(shí).
如：He did it as if he were an expert.
Even if she were here,she could not solve the problem
if
連接詞 conj.
1.(表示條件)如果
He will come if you invite him.
如果你請(qǐng)他,他會(huì)來(lái)的.
2.(表示虛擬)假如,要是
If I were you,I would never do that.
如果我是你,我決不做那件事.
3.(表示讓步)即使
We'll go even if it rains.
即使下雨我們也要去.
4.是否
I wonder if she is ill.
不知她是否病了.
5.(表示因果關(guān)系)每一次...的時(shí)候(總是會(huì))
If I feel any doubt,I inquire.
我一感到有疑問(wèn)就隨時(shí)詢(xún)問(wèn).
名詞 n.
1.條件;設(shè)想[C]
There are too many ifs in his offer.
他的提議中條件太多了.
IF=i.f.
if.
縮寫(xiě)詞 abbr.
1.=information feedback 信息回饋
2.=interferon
3.=intermediate frequency 中頻
i.f.
縮寫(xiě)詞 abbr.
1.=intermediate frequency 中頻
if從句的用法：
一,引導(dǎo)條件狀語(yǔ)從句,“如果,假設(shè)”的意思,
If you don't go there,I won't,either.如果你不去,我也不去.
二,引導(dǎo)賓語(yǔ)從句,“是否”的意思,如：
He asked if I could help him他問(wèn)我是否幫下他.
once：
做副詞
1.一次,一回.
eg：I saw him once and never again.我見(jiàn)過(guò)他一回,以后再也沒(méi)見(jiàn)到過(guò).
Once a week ,she wrote to her mother.她一星期一次給她媽媽寫(xiě)信.
2：曾經(jīng),一度
eg：She was oce my best friend.她曾是我最好的朋友.
3：用于條件句、否定句等,一旦
eg：If you once forget it,you won't come up with it again.你一旦忘記它,就不會(huì)再想起來(lái).
4：用于否定句,一次也,完全
eg：I haven't seen him here once.我自愛(ài)這里一次也沒(méi)見(jiàn)到過(guò)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：親屬關(guān)系上隔一代
eg：a cousin once removed 同（外）曾祖的堂（或表）兄弟（姐妹）
做名詞
一次,一回
eg：Do it just this once .就做這一次.
做連詞
一旦.就.
eg：Once you have taken the examination ,you will be able to relax.
你一旦考過(guò),就可以輕松了.
做形容詞
從前的,一度的
eg：the once capital of the nation 那個(gè)國(guó)家的舊都.