that：陈述句做宾语从句时，用 that 连接，that 在句中没有实际意义，不能充当句子成分，口语中可以忽略例句：I think (that) he will come at once. what:特殊疑问句做宾语从句时，用 what，when，who，which等引导例句：I don't understand what he really wants.
