Thread: Use "Must" for Present Simple deduction

1. Use "Must" for Present Simple deduction

Greeting folks,

I have been working on the modal verbs for a while, and there is a part of it, the deduction function, which is really interesting. I notice that whenever people use "Must" to deduce, there are always 3 ways to do it:
1) The present simple: Must be + Complement (e.g. He must be tired; She must be an CIA agent)
2) The present continous: Must be + V-ing
3) The past: Must have done

My question is, can we ever use "Must" to indicate a logical probability of a normal action of the present simple. Let's examine this dialogue to get my idea:
- I need to bring my kid to hospital for vaccine.
- Oh I know a good place for him. I have a friend, a nurse, at ABC Hospital. He should be able to help you.
- Really. But can he inject gently?
- Of course he can. He must know how to do it gently. After all, that is his job, isn't it. He has been doing it for 10 years, you know.

I personally think that we can use "Must" for the case above, and in this context it should not be misunderstood with obligation or necessity. Howerver, I have done many researches online, and none of the results has shown "Must + V" for deduction so far except for the three at the beginning of my post. Therefore, could I use "Must" like in the example, or do I have to change it to "Have/has to", or are there any other ways to deduce a normal action in the present simple?

Many thanks!

2. Re: Use "Must" for Present Simple deduction

Sure we can. This is actually a very common use of must. Just as your hypothetical nurse "must know", here is a current example of the negative form of the usage:

Anyone who opposes vaccination must not know how very useful vaccines are.

3. Re: Use "Must" for Present Simple deduction

You don't know how grateful I am right now receiving your reply. Thank you!

Still it is strange for me seeing you wrote that that is "a very common use" when no result I have searched so far tell the same thing. And I am even more stunned by your example of the negative form. I thought it should only be used for obligation, to restrict somebody from doing something? But yes, looking back to all the deduction strutures, I have to admit that there is no one that can be used to express a negative deduction except for "Must not". Sometimes you have to reach an expert just to get the answer you need.

Again, thank you so much!

4. Re: Use "Must" for Present Simple deduction

Oh and by the way, "Should" can replace "Must" for deduction purpose if my certainty level is lower, can't it?

For example:
...
- Really. But can he inject gently?
- I... I think so. He's a nurse, right? He should know. (He probably knows how)

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