EXPRESSING DEGREES OF CERTAINTY: PRESENT TIME

Examples:

--Why isn't John is class?

100% sure: He is sick.

95% sure: He must be sick.

Less than 50% sure:

He may be sick.

He might be sick.

He could be sick.

"Degree of certainty" refers to how sure we are--what we think the chances are--that something is true.

If we are sure something is true in the present, we don't need to use a modal. For examples, if I say, "John is sick," I am sure; I am stating a fact that I am sure is true. My degree of certainty is 100%.

--Why isn't John in class?

(a) He must be sick. (Usually he is in class every day, but when I saw him last night, he wasn't feeling good. So my best guess is that he is sick today. I can't think of another possibility.

Must is used to express a strong degree of certainty about a present situation, but the degree of certainty is still less than 100%.

In (a) The speaker is saying: "Probably John is sick. I have evidence to make me believe that he is sick. That is my logical conclusion, but I do not know for certain.

--Why isn't John in class?

(b) He may be sick.

(c) He might be sick.

(d) He could be sick. (I don't really know. He may be at home watching TV. He might be at the library. He could be out of town.)

May, might and could are used to express a weak degree of certainty.

In (b), (c), and (d): The speaker is saying: "Perhaps, maybe, possibly John is sick. I am only making a guess. I can think of other possibilities.

(b), (c) and (d) have the same meaning.

Maybe  (spelled as one word) is an adverb. Maybe he is sick. 

May be (spelled as two words) is a verb form: He may be sick.

Naeem

Cheers!

Source(s):
Understanding and Using English Grammar by Betty Schrampfer Azar