Which one is correct?

Hi everyone,recently i am confused by a grammar problem.I hope someone could help me and I will be appreciated to you.
Which of the two types of sentence below is correct or they are both correct?
a:We had just had our breakfast when Tom came in.
b:We have just had our breakfast when Tom came in.
and here is another group
a:It must had been about two in the morning when I returned home.
b:It must have been about two in the morning when I returned home.

[Casiopea]

a: We had just had our breakfast when Tom came in.

b: It must have been about two in the morning when I returned home.

Thank you,Casiopea .
Can you tell me why?

[Casiopea]

Originally Posted by richard001

Thank you,Casiopea .
Can you tell me why?

Re: We had just had our breakfast when Tom came in.

Event #1: had breakfast => had had breakfast (past perfect)
Event #2: Tom came in => came in (simple past)

The underlined portion functions as modification:

We had already/just had breakfast when Tom came in.

Re: It must have been about two in the morning when I returned home.

'must have' is a set phrase. That is, there isn't a 'must had' phrase.

For the 2nd sentence,if there was not a 'must',it had been about two in the morning when Ireturned home,is correct?

[Casiopea]

Originally Posted by richard001

For the 2nd sentence,if there was not a 'must',it had been about two in the morning when Ireturned home,is correct?

Well, 'two in the morning' is not an event, and there's no need to use the past perfect if there's only one event:

EX: I was two in the morning when I retuned home.

Two events
My mom had been waiting for me when I got home at two in the morning.

I am clear now,thank you Casiopea.

[Casiopea]

Great. :D