if和whether都可譯為\"是否\",二者引導(dǎo)賓語(yǔ)從句時(shí)通?？梢曰ハ嗵鎿Q,口語(yǔ)中多用if代替whether.例如：Ask if/whether he plans to come to the meeting.而以下幾種情況中,只能用whether:
1.強(qiáng)調(diào)兩方面的選擇,特別是句中有or not時(shí).如：
Let me know whether you can come or not.
2.賓語(yǔ)從句前置時(shí).如：
Whether this is true,I can\'t say.
3.引導(dǎo)主語(yǔ)、表語(yǔ)、同位語(yǔ)從句時(shí).如：
Whether the football game will be played depends on the weather.（主語(yǔ)從句）
The question is whether he can do it.（表語(yǔ)從句）
The question whether we should go with them must be decided at once.（同位語(yǔ)從句）
4.在介詞之后作介詞的賓語(yǔ)時(shí).如：
I haven\'t settled the question of whether I\'ll go back home.
5.在不定式前與不定式一起組成詞組時(shí).如：
Whether to go or stay is still a question.
6.在discuss,decide 等動(dòng)詞后作賓語(yǔ)時(shí).如：
We discussed whether we should go there by plane.
值得注意的是：if還可以引導(dǎo)條件狀語(yǔ)從句,是\"假如；如果\"之意,而whether可以引導(dǎo)讓步狀語(yǔ)從句,是\"不管；無論\" 之意.如：
If you ask him,he will help you.
I shall go,whether you come with me or stay at home.
一、if或whether引導(dǎo)賓語(yǔ)從句時(shí),作“是否”講,常放在動(dòng)詞ask 、see 、say 、know和find out等后面.一般情況下,兩者常可換用,在口語(yǔ)中多用if.如：
例1 Nobody knows whether (if) it will rain tomorrow.沒有人知道明天是否下雨.
例2 Lucy asked whether (if) they had a cotton sweater .露西問他們是否有棉制的羊毛衣.
二、if或whether引導(dǎo)賓語(yǔ)從句時(shí),要注意三個(gè)方面,即連詞、語(yǔ)序和時(shí)態(tài).
1 if或whether不能和that 或其它連詞（副詞）同時(shí)使用,也不能省去.如：
例3 我不知道他今天是否會(huì)來.
[誤] I don’t know that if(whether) he will come here today .
[正] I don’t know if(whether) he will come here today .
例4他走過來看看貓是否出了毛病.
誤] He came to see what if (whether) there was wrong with his cat .
[正] He came to see if (whether) there was wrong with his cat .
2 if或whether引導(dǎo)的賓語(yǔ)從句時(shí),雖具有疑問意義,但從句語(yǔ)序應(yīng)用陳述句語(yǔ)序.如：
例5 Did you know the way to the hospital The old woman asked me .
The old woman asked me .if (whether) I knew the way to the hospital .老婦人問我是否知道去醫(yī)院的路.
3 if或whether引導(dǎo)的賓語(yǔ)從句應(yīng)和主句的時(shí)態(tài)保持一致.即主句為一般現(xiàn)在時(shí),從句用任一種時(shí)態(tài)；主句為一般過去時(shí),從句則用過去時(shí)中的任一種時(shí)態(tài).如：
例6 I don’t know if (whether) he has come here .我不知道他是否來過這兒.
例7 Tom asked if (whether) I had read the book .湯姆問我是否看過這本書 .
三、if和whether的區(qū)別：
1 在動(dòng)詞不定式之前只能用whether .如：
例8 I can’t decide whether to stay.我不能決定是否留下.
2 在whether …… or not 的固定搭配中.如：
例9 I want to know whether it’s good news or not .我想知道是否是好消息.
3 在介詞后,只能用whether.如：
例10 His father is worried about whether he lose his work .他的父親擔(dān)心是否會(huì)失去工作.
4賓語(yǔ)從句放在句首表示強(qiáng)調(diào)時(shí),只能用whether .如：
例11 Whether they can finish the work on time is still a problem .他們是否能準(zhǔn)時(shí)完成這項(xiàng)工作還是個(gè)問題.
5用if會(huì)引起歧義時(shí),只用whether.如：
例12 Could you tell me if you know the answer
這句話有兩種意思：“你能告訴我是否知道答案嗎?”或“如果你知道答案,請(qǐng)告訴我,好嗎?”.如用whether可避免歧義.