你对must be 和must have been 用法的理解是对的，但“总是分不清”，说明掌握不透彻，不能灵活运用。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。

Sorry I made such a fool of myself last night. I must have been drunk. 对不起，我昨晚真是出丑了，我一定是喝醉了。

It must have been very painful for you to tell her about the accident. 把这次事故告诉她对你来说一定是很痛苦的事。