He has great calves. He must run very fast.

is a strong inference using 'must'.

Can the sentence using 'can't' always have the opposite meaning of 'must' (inference)?

You said he's faster than an ostrich but look, he's limping his leg.
He can't run that fast.

I'm not familiar with the above expression as the ones below.

He must be a good runner
That must be true!

He can't be a good runner.
That can't be true!

Am I using all the expressions correctly?

  • The plural of "calf" is "calves". It's one of those irregular nouns.
    – Andrew
    Jan 13, 2018 at 3:40

1 Answer 1


Let me state up front that the plural of calf is calves. See also half, halves, hoof, hooves, and so on. Very often a noun ending in f will change to a v for the plural and add -es.

Also, we just say limping, not limping his leg. We already know it's his leg that's limping.

Now, must denotes requirement. If you must do something, you are required to do it. Can denotes ability. If you can't do something, you're unable to do it. But when using must and can't in the way that you are using them, they are expressing conclusions that you are drawing. Must is an affirmative conclusion; can't is a negative conclusion.

Suppose person A tells person B this:

Joe covered (i.e. traveled) five blocks in three minutes. He must be a good runner.

Person A is saying that the evidence suggests to him that Joe is a good runner.

Now, can't draws the opposite conclusion. Suppose person B has evidence that suggests that person A has drawn the wrong conclusion:

B: Joe can't be a good runner, at least he can't be today. I saw him limping this morning. He must have sprained his ankle. Maybe he has a bicycle.
A: Well, he must have, because I know he doesn't have a car.

B is saying that other evidence suggests that Joe is not a good runner. He then gives an alternative possibility. A then says that he agrees with the conclusion that Joe has a bicycle.

Keep in mind that in casual conversations like this one, people using must and can't often understand that their conclusions are speculative. Both A and B know that they might not have the evidence necessary to prove their positions. Whether or not the conclusions are speculative depends on context:

A: Joe and Harry look like brothers.
B: Well, Harry is the oldest kid in his family, and Joe is older than Harry, so they can't be brothers.
A: I guess not.

Finally, other than the corrections I gave you up top, you are using the expressions correctly. Except one thing:

Whoever said he was faster than an ostrich can't be serious. Nobody can run as fast as an ostrich, whether limping or not.

Here is an example of how fast ostriches can run, for your amusement.

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