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(méi)有人知道明天是否下雨.
例2 Lucy asked whether (if) they had a cotton sweater .露西問(wèn)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(lái).
[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(guò)來(lái)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(wèn)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(wèn)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(guò)去時(shí),從句則用過(guò)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(lái)過(guò)這兒.
例7 Tom asked if (whether) I had read the book . 湯姆問(wèn)我是否看過(guò)這本書(shū) .
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(wèn)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.