must have ,必须有,必须要,肯定已经.
过去时.i must have done that before.我肯定干过那事.
i must have it.我一定要得到,一般现在时.
现在完成时是BE动词家过去分.must have的现在完成时在日常生活中很少出现.硬翻译过来也不是不可能.举个例子：i am must had dinner,我肯定刚吃完晚饭.一般正常的说法是:i am done with dinner.我吃完饭了.
过去时：i've seen you before.我见过你.i've been there before.意思是我去过那,但此时此刻本人并不在那.
完成时：i had been here.我来过这,本此时此刻也在这.