Learn English – “would” vs “must” in drawing logical conclusion or presumption

grammar

One of the use of would has been listed below:

You use would , or would have with a past participle, to indicate that you are assuming or guessing that something is true, because you have good reasons for thinking it.

⇒ You wouldn't know him.

⇒ His fans would already be familiar with Caroline.

⇒ That would have been Della's car.

⇒ He made a promise to his great-grandfather? That would have been a long time ago.

⇒ It was half seven; her mother would be annoyed because he was so late.

Source :http://www.collinsdictionary.com/dictionary/english/would

What I'm most confused(not understood) is as follows:

Use of would is just like the use of 'must' for drawing logical conclusion or presumption: for example:

*His fans would already be familar with caroline.

You mustn't know him.That must have been Della's car.

So, how would is different from 'must' in this usage ?

I would appreciate your helpful responses.Thank you.

Best Answer

In grammar, the idea of modality is to express subjective attitudes and opinions of the speaker about a possibility, probability, necessity, obligation, permission, ability, desire, and contingency.

All these categories can be ranked by the degree from high to low, where must refers to the former and would — to the latter. In other words, must asserts what we conclude to be the most likely interpretation of a situation or events, and would — a less likely one.

Cut to the chase, compared with would, must expresses a higher degree of probability and means "I'm sure", "I'm most certain", "most probably" whereas would means "I suppose/assume", "probably", "maybe", "I should think".

To refer to the past, we use would with the perfect infinitive:

A: I met a most beautiful girl at John's party yesterday.

B: Ah, yes! That would have been his cousine Ann.

P.S. Should you want to have modal verbs listed by their modality strength approximately, here you are:

High modality: must, ought to, shall, has to;

Medium modality: will, should, can, need to;

Low modality: may, might, could, would.

You should bear in mind that the idea of modality can also be expressed through nouns, adjectives and adverbs:

Modal nouns: possibility, probability, certainty, obligation, necessity, requirement;

Modal adjectives: possible, probable, obligatory, necessary, required, determined, likely, certain;

Modal adverbs: possibly, probably, maybe, perhaps, sometimes, always, never, certainly, definitely.

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