(1) He must regret his decision.
(2) He must be regretting his decision.
If you’re sure that "he" regrets his decision, can you say either (1) or (2)?
In other words, can (2) be interpreted as not having a progressive meaning to it?
To answer your last question first, it will always be interpreted as having a progressive meaning, that doesn’t change. Sometimes, though, that distinction isn’t very meaningful in describing certain situations. In this example, you can say either 1 or 2 and as a practical matter, the difference in meaning is just a nuance.
Example 1 might describe someone who regrets the decision, but it is not a continuous emotion, the regret is only when he happens to think about it. Example 2 describes someone who is actively thinking about the decision and regretting it. The “continuing” aspect does describe something that is different, but it is only a subtle difference in this case.
Your comment mentions another example: “What do you do?” vs. “What are you doing?” That’s a case where the two forms result in an entirely different meaning. The degree to which the progressive form describes something substantially different in real life is very much a function of the specific word and the context.
You added an interesting additional question in a comment: can (1) also mean “I’m sure that he will regret his decision in the future” instead of “I’m sure that he regrets his decision now”? Wording your question that way, which is consistent with the comparative meanings in the original question, the answer is no. You used “must” in the sense of “it is likely”.
However, a different meaning of “must” could turn it into a future reference. If you use “must” to mean a requirement or obligation, “he must regret his decision” could refer to an action expected in the future. But that’s different from what you asked.
Source : Link , Question Author : JK2 , Answer Author : fixer1234