Grammar : Modal Auxiliaries : Might, Should, Must, Can't , Could + have been + (verb 2 +-ing)

Learn Grammar | Grammar : Modal Auxiliaries : Might, Should, Must, Can't , Could + have been + (verb 2 +-ing) | 1. Might + Have been + verb + -ing
Modals are used to express a presumption against a thing, which perhaps is in progress in the past.
I saw him standing in front of the street. He might have been waiting for the bus.
She might have been sleeping when I came.
She might not have been taking a bath.

2. Must + had been + verb + -ing
The use of capital is basically the same as the use of modals; must have been + verb-ing. The difference is that the modals; Must Had + been + verb-ing, there is information indicating a specific time period.

He must had been playing the piano for an hour when I saw him.

3. Should + had been + verb ing
Similarly, the use of these modals is the same as the use of modals Should have + been + verb-ing, where the difference is the presence of time information indicating a certain period during a particular activity at the time anyway.

They should have been working for two hours when their boss came.
The students should had been doing their exercise. 
 http://learngrammar.blogspot.com/