Wilber: Someone's knocking.
Gertrude: That must be Sydney. (HIGH CERTAINTY)
That will be Sydney.
That should be Sydney.
That may be Sydney.
That could/might be Sydney. (LOW CERTAINTY)
The above is an excerpt from the The Grammar Book. I don't think I completely agree with the scale of probability presented above and I have some more doubts related to this use of modals. To me, they are quite subtle points so I'm not even sure I'll be able to get them across. Please feel free to enlighten me!
First of all, I'm a learner and I'm more exposed to American English and I believe that to my ear the sentence with will sounds more certain than the one with must. Moreover, That would be Sydney would sound better than That will be Sydney.
Secondly, the choice of modals of probability seems to depend more on situations than on the scale itself. For instance, let's say that I know Sydney very well (Sydney being my daughter's friend) and we've been expecting Sydney because she called to come over, I'd say, "that should be Sydney." On the other hand, I haven't met Sydney but have heard of her and when I answered the door, there's a Sydney-looking girl on the porch asking for my daughter then I would say, "you must be Sydney." If Sydney has been regularly visiting us around this time and I hear someone knocking on the door, then I would say, "that would(will) be Sydney."
Thirdly, while can can be used in the negative and in questions to express probability, I wonder why can can not be used in the affirmative. For instance, "that can't be Sydney" and "can it be Sydney?" sounds OK but "that can be Sydney" sounds not quite right unless it means something potential as in "that can be tricky." Any comments on these? Thanks in advance!
Last edited by yuriya; 19-Jul-2010 at 06:10.
By using "will, would, should, must, is probably", the probability is likely to be greater than 50% - it is more probable than not that it is Sydney (if the person speaking is a competent judge of probability).