是否英语的用法

whether和if是中学英语中常见又常用的两个连接词,二者都可以表示“是否”.现将它们的用法归纳如下： whether和 if 在 引导宾语从句时常常可以互换.如： I don’t know whether ／if he will be able to come． 我不知道他是否能来. 但在下列情况下常常采用whether而不用if： 1．宾语从句中提出两种选择时： He doesn’t know whether he should go with me or stay at home．他不知道是该和我去还是留在家里. Tell me whether he is at home or not． 告诉我他是否在家. 2．宾语从句前置时： Whether she will come,I’m not sure．我不确定她是否来. 3．引导介词后的宾语从句时： I worry about whether I hurt her feelings．我担心是否伤了她的感情. It all depends on whether they will support us．这完全取决于他们是否支持我们. 4．为避免产生歧义,常用whether表示“是否”. Please let me know whether you need my help．请告诉我你是否需要我的帮助.（表示此意时,此处不宜用if） Please let me know if you need my help． 此句有两种含义：(1) 请告诉我你是否需要我的帮助.(2) 如果需要我的帮助,请告诉我. 5．引导主语从句、表语从句或同位语从句时： Whether he will come is uncertain． 他是否来还没有确定. The question is whether it is worth doing．问题在于做这事是否值得. The problem is whether he has the ability to do the job．问题是他是否有干这工作的能力. The question whether there is life on that planet remains open．究竟在那个行星上是否有生命,这个问题尚无定论. 6．后接不定式短语时： He doesn’t know whether to go or not．他不知道是去还是不去. She hasn’t decided whether to accept or refuse．她还没有决定是接受还是拒绝. 7．引导让步状语从句时： Whether he agrees or not, I shall do that． 不管他同意与否,我都要做. I show everything on my face, whether I am angry or pleased． 无论生气还是高兴,我都会表现在脸上. 注意：引导条件状语从句时,用if,译为“如果,假定”. If it rains,we’ll stay at home．如果下雨,我们就待在家.

如果;if 用作为如果，常用作为主将从现。如：We'll go to the supermarket if it doesn't rain(但请你注意：1.主句可以用祈使句来表示将来时，这时主句用一般现在时 Put on your coat if you feel cold 2.是真理的话，前后都用一般现在时)解释为是否时，用法同whetherE.g.: I dont't if he has passed exam也可以用whether ,前后时态统一用主将从现的还有：as soon as, when, before, until,unless，after……这类词有很多情况都是用过去时的，但若主句是用将来时，从句就用一般现在时如：I'll call you as soon as he comes back.其他的也要注意主句的时态就可以判断了

1.(表示条件)如果 ,主句用将来时,if从句用现在时表示将来He will come if you invite him.如果你请他,他会来的.2.(表示虚拟)假如,要是 1）时态：可以表示过去,现在和将来的情况.它的基本特点是时态退后.a.同现在事实相反的假设.句型 ：条件从句 主句 一般过去时 should( would) +动词原形If they were here,they would help you.b.表示于过去事实相反的假设.句型：条件从句 主句 过去完成时 should(would) have+ 过去分词If she had worked harder,she would have succeeded.The rice would not have been burnt if you had been more careful.If my lawyer had been here last Saturday,he would have prevented me from going.If he had come yesterday,I should / would have told him about it.含义：He did not come yesterday,so I did not tell him about it.If he had not been ill and missed many classes,he would have made greater progress.含义：He was ill and missed many lessons,so he did not make greater progress.c.表示对将来的假想句型：条件从句 主句一般过去时 should+ 动词原形were+ 不定式 would + 动词原形should+ 动词原形If you succeeded,everything would be all right.If you should succeed,everything would be all right.If you were to succeed,everything would be all right.3.是否 = whether 连接宾语从句I wonder if she is ill.不知她是否病了.