①賓語(yǔ)從句是否定句時(shí)，只用if，不用whether。
e.g. I wonder if it doesn’t rain.
②用if 會(huì)引起誤解，就要用whether。
e.g. Please let me know whether you want to go（此句如果把whether改成if，容易當(dāng)成條件句）?
③賓語(yǔ)從句中的whether 與or not直接連用，就不能換成if；不直接連用，可換。?
e.g. I don’t know whether or not the report is true. I don’t know whether/ if the report is true or not.
④介詞后的賓語(yǔ)從句要用whether引導(dǎo)。whether 可與不定式連用。whether也可引導(dǎo)主語(yǔ)從句、表語(yǔ)從句、同位語(yǔ)從句，還可引導(dǎo)讓步狀語(yǔ)從句，以上均不能換成if。但引導(dǎo)條件從句時(shí)，只能用if，而不能用whether。
e.g. It depends on whether we have enough time. They don’t know whether to go there. Please come to see me if you have time.
注意：在demand、order、suggest、decide、insist, desire, request, command, doubt等表示要求、命令、建議、決定等意義的動(dòng)詞后，賓語(yǔ)從句常用“（should）+ 動(dòng)詞原形”。
e.g.I insist that she (should) do her work alone. 我堅(jiān)持要她自己工作。
? ? ?The commander ordered that troops (should) set off at once. 司令員命令部隊(duì)馬上出發(fā)。