if引導(dǎo)兩種條件句：真實條件句和虛擬條件句.
真實條件句：
If he has written the letter,I'll post it.
If I have time(表示條件的是從句),I'll come over to see you.（由條件導(dǎo)致的結(jié)果就是主句）
在真實條件句里,從句用一般現(xiàn)在時表示將來時,主句用一般將來時.
虛擬條件句：
If I had enough money（從句）,I would buy a computer（主句）.
如果我有錢,我要買一臺電腦.（但我沒有）
虛擬語氣用于表達(dá)不可能實現(xiàn)的事情.虛擬條件句又有現(xiàn)在情況,將來情況,和過去情況.
1）表示現(xiàn)在或?qū)恚?br/>條件從句中的謂語是相當(dāng)于過去時的形式,主語謂語用would+動詞原形：
If I had a map I would lend it to you.如果我有地圖我就借給你.（但我沒有）
If I dyed my hair blue everyone would laugh at me.如果我把頭發(fā)染成藍(lán)色,誰都會笑我的.（當(dāng)然我不會干這樣的傻事）
2）表示過去：
條件從句中的謂語用過去完成時,主句的謂語動詞要用would have＋過去分詞 ：
If I had known that you were coming,I would have met you at the airport.如果我知道你要來,我就去機(jī)場接你了.
If anything had happened,he would have let us know.如果發(fā)生了什么情況,他在就通知我們了.