1.当主句中存在动词do的各种形式(does、did、have done、had done、will do、to do),介词but 后就省去to; 例如:I like to do nothing but swim today. 我今天只愿意游泳。 I did nothing but sleep. 我一直在睡觉。 I had nothing to do but watch TV. 我只能看电视了。 原句也可改为:I want to do nothing but go to the beach. 2.当主句中是其他动词,而没有do的任何形式时,but后的动词前就要加to; 例如:I had no choice but to keep silence. 我只能保持沉默。 He said nothing but to leave. 他悄无声息的离开了.