must have been 和 must be的区别在 have
have在这里表示是 动作已经完成.所以
must have been是对过去的事实的肯定猜测
must be是对现在事实的肯定猜测
e.g.
She sings so well.I think she must have been a singer.
她唱得真好.我想她以前肯定是名歌手.
he doesn't come to school today.He must be ill.
他今天没有来上学,肯定是病了.