In order to take that job,you must have left another job.
为了去做那份工作,你肯定已经辞去了另一份工.
（像这个句子,后面的动作是前面的额动作发生的前提）
You use must to indicate that you are fairly sure that something is the case.肯定 (表示猜测）
看这里的句子,表示最大可能的猜测
It must have stopped raining.雨很可能停了.
I must have been sleeping .应是对过去的时候的猜测
我那时候一定是睡着了,be sleeping睡着了,是一种状态,睡着的状态
你的是对的