Simone
简单地说：对过去情况作推测，用must have been；所对现在情况作推测，用must be。
比较：
He must be ill. 他一定是病了。（指现在有病）
He must have been ill. 他当时一定是有病 / 他一定生过病。（指过去有病）
He must be very rich. 他一定非常富有。（指现在有钱）
He must have been very rich. 他当时一定非常富有 / 他一定曾经富有过。（指过去富有过）
下面是几个must have been的例子，有一定的背景或语境，你可以体会一下：
She must have been very young when she got married. 她结婚时一定很年轻。
I didn't hear the phone. I must have been asleep. 我没有听到电话铃，我一定是睡着了。
When you got lost in the forest you must have been very frightened. 当你在森林里迷路的时候，你一定很害怕吧。
I must have been blind not to realize the danger we were in. 我一定是瞎了眼，连我们身临险境也看不出来。
这下你明白must be与must have been 的区别了吗？该不会再误用must have been与must be了吧！
都表推测，只不过must have been 表示对过去发生的推迟。
