if和whether都可譯為\"是否\",二者引導賓語從句時通常可以互相替換,口語中多用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.強調兩方面的選擇,特別是句中有or not時.如：Let me know whether you can come or not.2.賓語從句前置時.如：Whether this is true,I can\"t say.3.引導主語、表語、同位語從句時.如：Whether the football game will be played depends on the weather.（主語從句） The question is whether he can do it.（表語從句） The question whether we should go with them must be decided at once.（同位語從句） 4.在介詞之後作介詞的賓語時.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片語時.如：Whether to go or stay is still a question.6.在discuss,decide 等動詞後作賓語時.如：We discussed whether we should go there by plane.值得注意的是：if還可以引導條件狀語從句,是\"假如；如果\"之意,而whether可以引導讓步狀語從句,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引導賓語從句時,作“是否”講,常放在動詞ask 、see 、say 、know和find out等後面.一般情況下,兩者常可換用,在口語中多用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製的羊毛衣.二、if或whether引導賓語從句時,要注意三個方面,即連詞、語序和時態.1 if或whether不能和that 或其它連詞（副詞）同時使用,也不能省去.如：例3 我不知道他今天是否會來.[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引導的賓語從句時,雖具有疑問意義,但從句語序應用陳述句語序.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院的路.3 if或whether引導的賓語從句應和主句的時態保持一致.即主句為一般現在時,從句用任一種時態；主句為一般過去時,從句則用過去時中的任一種時態.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的區別：1 在動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 .我想知道是否是好訊息.3 在介詞後,只能用whether.如：例10 His father is worried about whether he lose his work .他的父親擔心是否會失去工作.4賓語從句放在句首表示強調時,只能用whether .如：例11 Whether they can finish the work on time is still a problem .他們是否能準時完成這項工作還是個問題.5用if會引起歧義時,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,請告訴我,好嗎?”.如用whether可避免歧義.
if和whether都可譯為\"是否\",二者引導賓語從句時通常可以互相替換,口語中多用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.強調兩方面的選擇,特別是句中有or not時.如：Let me know whether you can come or not.2.賓語從句前置時.如：Whether this is true,I can\"t say.3.引導主語、表語、同位語從句時.如：Whether the football game will be played depends on the weather.（主語從句） The question is whether he can do it.（表語從句） The question whether we should go with them must be decided at once.（同位語從句） 4.在介詞之後作介詞的賓語時.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片語時.如：Whether to go or stay is still a question.6.在discuss,decide 等動詞後作賓語時.如：We discussed whether we should go there by plane.值得注意的是：if還可以引導條件狀語從句,是\"假如；如果\"之意,而whether可以引導讓步狀語從句,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引導賓語從句時,作“是否”講,常放在動詞ask 、see 、say 、know和find out等後面.一般情況下,兩者常可換用,在口語中多用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製的羊毛衣.二、if或whether引導賓語從句時,要注意三個方面,即連詞、語序和時態.1 if或whether不能和that 或其它連詞（副詞）同時使用,也不能省去.如：例3 我不知道他今天是否會來.[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引導的賓語從句時,雖具有疑問意義,但從句語序應用陳述句語序.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院的路.3 if或whether引導的賓語從句應和主句的時態保持一致.即主句為一般現在時,從句用任一種時態；主句為一般過去時,從句則用過去時中的任一種時態.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的區別：1 在動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 .我想知道是否是好訊息.3 在介詞後,只能用whether.如：例10 His father is worried about whether he lose his work .他的父親擔心是否會失去工作.4賓語從句放在句首表示強調時,只能用whether .如：例11 Whether they can finish the work on time is still a problem .他們是否能準時完成這項工作還是個問題.5用if會引起歧義時,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,請告訴我,好嗎?”.如用whether可避免歧義.