"They SHOULD have been there."

"They should have been there." means "Too bad that they were not there, it was such a good show or something like that." However if you put "by now" behind it. It would become "They should have been there by now," which means "Well, they are probably there now or they probably have gotten there now."

Therefore, I create some sentences below using the same logic, would you check them for me?

1. They COULD have been there by now.

It means they are probably there now.

2. They MIGHT have been there by now.

It means the same.

3. They MAY have been there by now.

It means the same, but even more definite than the above two.

4. They MUST have been there by now.

It means they are very likely there now, like 90%.


Well, actually I am so not sure about this theory; I even doubt the beginning statement (the "should have" part). So, how do you think? Is it right? Or is it a total mistake or although it's a mistake but some Americans keep using unwittingly? Like they doubt it, but they keep saying it or think it's ok but they wouldn't say it's right.

Thanks!

Your original example actually implies that they ought to have arrived, but that they have not, or that the speaker does not know if they have but suspects they have not. Perhaps they promised to arrive at 12:30 PM, but the speaker just got a phone call saying it's 1 PM and they have not arrived.

1 really means that if they had done something else, such as leave earlier, not get lost, or take a better means of transit, they would have been able to get there by now, but they are in fact not there yet.

2 is similar, I guess... if they had done whatever it was, they would maybe have gotten there by now. Or maybe not.

For one and two, you might want to add an if... statement, such as:
If only they had left sooner, they could have gotten there by now.

3 doesn't work. Try 'they may be there by now' or 'they may have gotten there by now'
The speaker suspects that it's possible that they have arrived, but is unsure.

4 doesn't work. Try 'they must have gotten there (or arrived there) by now' or 'they must be there by now'
The speaker here is certain that 'they' have arrived.

3 and 4 seem to imply that 'they' (may/must) have been there, but that they left and are now somewhere else. It's kind of a weird, awkward construction in general.

For the record, I am American, from the west coast of the United States.