"would" vs "must" in drawing logical conclusion or presumption

Question

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.

Answer 1

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.

Answer 2

A basic relation that can be expressed by both of these is "Because of B, A X" where X various things including is "can't do B" or "wants to do B" or even "wants to do B but can't." So both of these are natural to express logical conclusions. The difference is outlined below.

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A must B means an external something (that is not A) is causing B.

This can be used to express an actual external something causing something else.

I must go to the hospital because I am sick.

The something can be an event/state (like above) or a logical conclusion:

Well he must turn left because that's the only way the road goes, unless he's going to drive on the grass.

It is also used to express an assumed external something causing something else - this is often done for politeness.

You must not know him. Let me introduce you. (The speaker is implying that it is not your fault you do not know him because an assumed external something that is not "you" is causing you not to know him.)

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A would B means A wants to do B but something is preventing A from doing B.

That something can be a specific thing blocking A from doing B,

I would go to the park but it's closed.

or it can be the fact that the circumstances are not right for A to do B, e.g. A is waiting for something before B can happen. Or, B does not exist yet.

I'm going to make this cake, I think Sally would like it. (She is blocked from currently liking it because the cake does not exist yet.)

Answer 3

There are a few differences between the two. "You must know Joe" is a lot stronger than, "You would know Joe." Must adds certainty. "You wouldn't know him" could be used in conversation like this:

Alice is talking to Bob, and Charlie comes into the room. Charlie is a friend of both Alice and Bob, and knows most of their friends, but not all of them. Jim is not in town often.

Alice: Hey, did you hear about Jim?
Bob: Yeah, he sure has some interesting stories.
Charlie: Jim's the guy we went fishing with last weekend?
Alice: Oh no, you wouldn't know him. He's not around very often. You're thinking of Todd.

There is a context difference here between "you wouldn't know him," and "you must not know him." While "must not" requires stronger evidence, the difference between "you wouldn't know him" and "you must not know him" is this: "you must not know him" expresses surprise or certainty. Usually there is a connotation of surprise. If the above conversation were instead

Alice: Hey, did you hear about Jim?
Bob: Yeah, he sure has some interesting stories.
Charlie: I've never heard any of his crazy stories.
Bob: Oh. You must not know him.

"That would have been a long time ago" means that something can be logically assumed to have taken place a long time ago. In this case, "must" doesn't add much, but it suggests that evidence has been weighed. In general, "would" could be used as "I guess" where as "must" implies at least some evaluation of evidence.

Answer 4

I think the difference between "would" and "must" can best be explained with these two of your examples:

⇒ You wouldn't know him.

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

With the first, it's conceivable but unlikely that you would know him: but he's lived in Alaska all his life. "Have you been to Alaska? You have! Have you been to Anchorage? No way! Did you ever drop in to 'Stefan's'? You did? Did you see the 6'8" chef with the tattoos and large beard? That's him. I was wrong!"

With the second, if the great-grandfather had been dead for ten years, and he made the promise to his great-grandfather directly rather than (say) to his gravestone, then you could replace 'would' with 'must' - it is impossible any other way.

Answer 5

You wouldn't know him.

This is somewhat dismissive. A: Who is Jake? B: You wouldn't know him.

You must not know him.

This can mean that you don't know who he really is. A: I can't believe Jake hit his girlfriend. B: You must not know him.

That would have been Della's car.

This can be a past unreal conditional. A: I just saw a blue car get pooped on by a bird. B: That would have been Della's car, if he hadn't ridden his bike to work today.

That must have been Della's car.

This means that the speaker is speculating that the car in question is Della's car. A: I just saw a blue car speeding out of the parking lot. B: That must have been Della's car.