if引導(dǎo)的條件狀語從句的分類及虛擬條件句的判斷
1、可以把條件句分為兩類：
1).真實(shí)條件句（Sentences of Real Condition）：凡是假設(shè)的情況發(fā)生性可能很大,就是真實(shí)條件句.例如：
⑴、If I have time ,I will help you with this work.如果我有時間會幫助你做此工作的.
⑵、As long as I say anything wrong,you must point it out.只要我說了什么錯話,你一定要指出來.
⑶、If time permits,we'll go fishing together.(如果有時間的話,我們就一起去釣魚.）
2).虛擬條件句（Sentences of Unreal Condition）：當(dāng)假設(shè)是不大可能實(shí)現(xiàn)時,就是虛擬條件句.例如：
⑴、If I were you ,I would have attended the meeting.如果我是你的話,就去參加會議了.
⑵、If he had come here yesterday,he would have seen his old friend.假如他昨天來這兒的話,就會看見他的老朋友.
⑴、If it had rained yesterday,we would have stayed at home.（如果昨天下雨的話,我們就會留在家里.）
2、.if條件狀語從句中虛擬語氣的判斷
判斷是真實(shí)條件句還是非真實(shí)條件句.只有在非真實(shí)條件句中才使用虛擬語氣.通過句子意思,看假設(shè)的條件是否能夠?qū)崿F(xiàn),能夠?qū)崿F(xiàn)是真實(shí)條件句,不能使用虛擬語氣；假設(shè)的條件不能實(shí)現(xiàn)則是非真實(shí)條件句,要用虛擬語氣.
判斷這個假設(shè)是與哪個事實(shí)相反.通常有三種情況：①與過去事實(shí)相反.②與現(xiàn)在事實(shí)相反.③與將來事實(shí)可能相反.
3、“后退一步法”
后退一步法是指在準(zhǔn)確地判斷了該句與哪一事實(shí)相反后,按虛擬語氣的后退一步法處理從句謂語動詞的時態(tài).即：在非真實(shí)條件狀語從句中,謂語動詞按正常情況“后退一步”.也就是：
①與過去事實(shí)相反,在從句中用過去完成時形式表示.
②與現(xiàn)在事實(shí)相反,在從句中用過去一般時形式表示.
③與將來事實(shí)可能相反,在從句中用過去將來時形式表示.
主句中則用情態(tài)動詞would,should,could 等加一個與從句一致的動詞形式.例：
⑴、If I had come her yesterday,I would have seen him.
⑵、If I were a teacher,I would be strict with my students.
⑶、If it should snow tomorrow,they couldn't go out.
4、注意事項(xiàng)
①if條件句中如有were,should,had,可以省去if,并使用倒裝語序.
②在現(xiàn)代英語中if條件狀與從句中的謂語動詞如果是be其過去形式一般用were.