if做連詞,主要有兩個(gè)意思,“是否”,引導(dǎo)賓語(yǔ)從句；“如果”,引導(dǎo)狀語(yǔ)從句：
一、if作為連詞,引導(dǎo)條件狀語(yǔ)從句,它表示的意思是“假如”“如果”等.在復(fù)合句中如果主句用將 來(lái)時(shí),則if引導(dǎo)的狀語(yǔ)從句用一般現(xiàn)在時(shí).例如：
1.If itdoesn’train,we will go to the park next Sunday.如果天不下雨,下周星期天我們將去公園.
2.If you ask him,he will help you．如果你求他,
if從句中用過(guò)去式,表示不可能實(shí)現(xiàn),大概不會(huì)實(shí)現(xiàn)或提出作為考慮的假定條件.此時(shí)需要用虛擬語(yǔ)氣,例如：
1.If you were a bird,you could fly.假使你是只鳥(niǎo),你便會(huì)飛了.
2.If I asked him(if I were to ask him) for a loan,would he agree?如果我向他借貸,他會(huì)答應(yīng)嗎?
另外,if從句中用過(guò)去完成式,表示過(guò)去未實(shí)現(xiàn)的條件(例如由于不可能實(shí)現(xiàn)或某人之未能實(shí)行).例如：
1.If they had startedearlier,they would have arrivedin time．要是他們?cè)缧﹦?dòng)身,他們便可及時(shí)到達(dá)了.
2.If they had not started when they did,they would not be here now．如果他們那時(shí)不動(dòng)身,現(xiàn)在他們就不會(huì)在此地了.
二、if還可以引導(dǎo)讓步狀語(yǔ)從句.這時(shí)if當(dāng)作“即使是”,“雖說(shuō)”解.例如：
1.If she’s poor,at least she’s honest． 雖說(shuō)她很窮,但至少她還是誠(chéng)實(shí)的.
2.If I am wrong,you are wrong,too． 即使說(shuō)我錯(cuò)了,那么你也不對(duì).
3.I’ll do it,even if it takes me all the afternoon． 雖然會(huì)花費(fèi)我一下午的時(shí)間,我還是要做這事.
三、if作為連詞還可以引導(dǎo)賓語(yǔ)從句.引導(dǎo)賓語(yǔ)從句時(shí)和whether意思相同.例如：
1.Lily asked if ／ whether she liked it.莉莉問(wèn)她是否喜歡它.
2.She asked if ／ whether they had a cotton one． 她問(wèn)是否他們有一件棉織的.