由连词if、 whether 引导的表示“是否…”的宾语从句.Whether,if 在从句中不做句子的成分,一般情况下,whether和if 可以替换.
例：I don’t know if/whether he will come tomorrow.
The teacher asked if/whether we had finished the experiment.
l在介词后面的宾语从句中不用if引导
例：Everything depends on whether we have enough money.
l宾语从句中有or not时不用if引导.
例：I don’t know whether the movie star will come or not.
l和不定式连用作宾语时不用if引导.
例：Whether to go there or not hasn’t been decided.