①宾语从句是否定句时，只用if，不用whether。e.g. I wonder if it doesn’t rain.②用if 会引起误解，就要用whether。e.g. Please let me know whether you want to go（此句如果把whether改成if，容易当成条件句） ③宾语从句中的whether 与or not直接连用，就不能换成if；不直接连用，可换。 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介词后的宾语从句要用whether引导。whether 可与不定式连用。whether也可引导主语从句、表语从句、同位语从句，还可引导让步状语从句，以上均不能换成if。但引导条件从句时，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、建议、决定等意义的动词后，宾语从句常用“（should）+ 动词原形”。e.g.I insist that she (should) do her work alone. 我坚持要她自己工作。     The commander ordered that troops (should) set off at once. 司令员命令部队马上出发。