He paid for a seat when he _____ have entered free.(could /would / must / need)请解释
问题描述：
He paid for a seat when he _____ have entered free.(could /would / must / need)请解释
答
could
这里是虚拟语气,本来是could have entered free免费入场了,但是他还是 paid for a seat.买了座位票．
表示和现实相反的虚拟用could