1

I still don't know what the difference between those perfect modals, especially between "might", "should" and "must".

  1. It was wrong of Glen to be so rude. He ______ (not behave) that way.
  2. Something ______ (happen) to Steve. He always calls when he's late.

Especially for sentence 1, why don't we use must? In my opinion, I think everyone who became rude is actually wrong. It's like "should" = something that you have to do/something right, but if you ignore, it doesn't matter. While "must" = Something that you have to do, and if you ignore it, you will face the consequences.

Why do I use "must"? Because, I think on this problem "It was wrong of Glen to be so rude.", we can see the consequences. Think about it, when someone being rude, no one want to see him nor talk to him. There is a consequences.

2

First, it is necessary to distinguish epistemic from deontic uses of the modals.

Epistemic uses are when the speaker is referring to their own assumptions or hopes about a situation not known for sure: "He must be there by now". "They couldn't have missed us, could they?" "I should be able to see him (when I go to town next wek)".

In this kind of use, the differences are mostly just of strength: "must" > "should" > "could" > "might/may".

Deontic uses are about the possibilities, potentialities and obligations in reality. "He must get there somehow", "They couldn't miss us: we were standing there waving". "I should be able to see him (now), but there's something in the way".

In this kind of use, the modals differ in quality as well as strength. "Must" and "should" are about obligation, or necessity, with "must" stronger than "should". "Can" and "could" (which mostly functions as a past of "can") are about ability and potentiality; and informally, about permission. "May" and "might" are about possibility, or uncertain intention; and "may" also about permission.

1

must is often used when it comes to an obligation made by the speaker. In the first sentence, you can use must but it's not an obligation. You use should in that case. Note that we want to show a disapproval of a past action, so we use should have, hence:

It was wrong of Glen to be so rude. He shouldn't have behaved that way.

In your second sentence, when you have the certainty about an action in the past, you use must have,

Something must have happened to Steve. He always calls when he's late.

1

First, all can be used in different contexts: as order/advice, or as suspicion.

must is stronger than should and should is stronger than might.

In the order/advice context:

Might is the weakest advisory: 'You might want to take an umbrella, the sky is clear, but the weather was tricky recently.'

Should is stronger:

'These clouds look really nasty. You should take an umbrella, it's going to rain soon.'

Must is 100% strength.

'It's raining cats and dogs, you must take an umbrella."

But they also appear as 'guessing.'

These fluffy clouds might be foreposts of a storm front.

With cloud cover this heavy, it should rain later today.

See that wall of falling water, whirring over downtown? It must be a tornado down there!

Caveats:

  • 'must' is very frequently used as hyperbole, often you'll encounter people using 'must' in contexts where 'might' would be the correct thing to say, just to express extra worry.

  • 'should' implies desire or expectation, not a fear. When you say "something might have happened to Steve" or "something must have happened to Steve" you're expressing a worried suspicion. You'll say "something should have happened to Steve" only if you hate Steve.

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