一、用法与词义
引导条件状语从句,意为“如果”;引导名词性从句,意为“是否”.有时若没有上下文可能产生歧义.如：
Write to tell me if you love her.
(1) 如果你爱她,请写信告诉我.
(2) 请写信告诉我你是否爱她.
二、时态问题
引导条件状语从句要用一般现在时表示将来,而引导名词性从句则没有此限制.如：
I don’t know if he will come.But if he comes,I’ll tellyou.我不知道他是否来,但是如果他来,我就告诉你.
有时表示条件的 if之后可能用 will,但那不是将来时态,而是表示意愿或委婉的请求(此处的 will是情态动词).如：
A：Shall I hold the door open for you?我为你开着门好吗?
B：Yes,if you will.好的,如果你愿意的话.
If you will wait a moment,I’ll fetch the money.请等一下,我就去拿钱.
三、省略问题
在 if 引导的条件从句里,有时可省略主语和谓语动词.在这类省略结构中,有的已构成固定结构,如：if any,if not,if necessary,if possible,if so 等.如：
I’ll help you if possible.若有可能我会帮你的.
There’re few,if any,mistakes in the book.这本书里即使有错也不多.
Fill in the blanks with articles if necessary.在有必要的地方填冠词.
If the weather is fine,we’ll go.If not,not.如果天气好,我们就去.如果天气不好,我们就不去.
He may be busy.If so,I’ll call later.If not,can I seehim now?他可能很忙.如果是这样,我以后再来拜访;他若不忙,我现在可以见他吗?
四、用于虚拟语气
若要表示与事实相反的假设,if引出的条件从句以及相应的主句则用虚拟语气,具体格式为:
(1) 与现在事实相反：主句用would,should,could,might+v.,从句动词用过去式(be 通常用 were).如：
If I were you,I might stay.我要是你,我可能会留下
If you had your choice,where would you go?要是让你选,你去哪儿?
(2) 与将来事实相反：主句用would,should,could,might+v.,从句动词用过去式(be 通常用were)或用 should+动词原形或 were to+动词原形.如：
If it rained tomorrow,I would stay at home.要是明天下雨,我就呆在家里.
If it should be necessary,I could come at six.如果有必要,我可以在六点钟来.
【注】从句谓语若为 should+动词原形,主句谓语有时可用陈述语气.如：
If he should call,tell him I’ll ring back.万一他打电话来,告诉他我给他回电话.
If I should see him,I’ll ask him to ring you.万一我能见到他,我就叫他给你打电话.
(3) 与过去事实相反:主句用would,should,could,might+have+过去分词,从句用过去完成时.如：
If he had worked harder,he would have succeeded.要是他更努力些,他就会成功了.
If it hadn’t been for the snow,we could have climbed themountain yesterday.要是没下雪,我们昨天就去爬山了.
(4) if only(但愿),as if(似乎,好象)也常与虚拟语气连用.如：
If only I knew her name.要是我知道她的名字就好了.
When a pencil is partly in a glass of water,it looks asif it were broken.当一支铅笔有一半放在水里时,它就好像折断了似的.
好了