what do you want for your birthday party?改为同义句.答案是what would you like for your birthday

答案没有错误.楼主理解有误,want不一定非要在后面加上to do sth表示“想要做某事”,也有want sth的结构.问句中的what即want后面的sth.且：want sth = would like sth,意思都是“想要（得到）某物”.再如：I want som...