1、宾语从句在复合句中作宾语.引导宾语从句的连词that一般可省略.
e.g.I hope (that) everything is all right.
2、介词之后的宾语从句,不可用which或if连接,要分别用what或 whether.
e.g.I'm interested in whether you've finished the work..
I'm interested in what you've said.
3、whether与if都可以引导宾语从句,常可互换.但下面情况不能互换.
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,容易当成条件句理解）
　　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.