They should be going by now. vs They should have gone.

[Hadeer]

Dear Alan,

What is the difference in meaning between these two sentences?

1- A: they should be going by now
B: they should have gone.

And these two sentences:

2- A: I’m very tired, I could sleep for a week.
B: I was very tired, I could have slept for a week.

Best wishes

Hadeer

[Gilson]

I think the only difference is the time that things happen.

Let´s see: A: they should be going by now (This is the time they are supposed to go.They should be leaving exactly at the moment you are talking!)
B: they should have gone. (It was expected they were no longer where they are,by the time you are saying.They should have gone,but something interrupted, causing some delay)

A: I’m very tired, I could sleep for a week. (I´m so tired,that i´d be able to spend the whole week sleeping.You are tired when you say it.(It´s still possible))
B-I was very tired, I could have slept for a week. (In this case,you WERE so tired,that you´d be able to sleep the whole week.(If you could,but you can´t))You could have slept,when you were tired,but you missed the opportunity)

I hope it helps you!!