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In the following situation, which fits best (should or must or have to):

I think that it is very important to look after the environment. This means that we should/must/have to cut down the amount of waste that we create.

I was taught that we use have to to express strong obligation, when the obligation comes from someone else.

And that we use must to express strong obligation, when the obligation comes from you.

And to use should to express mild obligation.

Accordingly, it is (MUST) that fits best, but I am still not sure about that.

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In my experience both "must" and "have to" are used for any strong obligation or need, ether coming from oneself or from an outside source; they can be used interchangeably.

The word "should" (and also the verb "ought to") is used when there is a legal or moral requirement, or when the thing is in some way the right or proper thing to do, but there is an option not to do it. The reason can still be strong, however. One might say "You should not commit a murder."

The Merriam-Webster definition of must shows several senses, and that of have to also has several senses but none suggest the distinction between external and internal source of obligation that I can see. The definition of should also has several senses, but one is "to express obligation, propriety, or expediency". Other dictionaries do not seem greatly different on these terms.

I do not agree with the meaniongs states in the question.

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As a rule, must and have to indicate a stronger level of obligation than should.

  • “Employees must attend the training session on workplace safety.”
    = Attendance at the session is an absolute requirement. If you don't go, you'll be fired.
  • “Employees have to attend the training session on workplace safety.”
    = As with must, this indicates a requirement, but to my ears have to sounds less forceful and more informal than must.
  • “Employees should attend the training session on workplace safety.”
    = We recommend/prefer that you go, but if something comes up and you can't go, we'll understand.
  • “Employees ought to attend the training session on workplace safety.”
    = To me, this sounds a bit milder than should. A suggestion.
  • “Employees may attend the training session on workplace safety.”
    = It's available if you're interested, but we don't really care whether you go or not.

So, the choice of which modal verb to use in a given context depends primarily on the severity of the consequences of not doing the thing.

  • If it would be unacceptable, use must.
  • If it would be inconvenient, use should.

“We must cut down the amount of waste that we create, before we turn the planet into a Wall-E-style giant landfill.” OK, that might be exaggerating, but you get the idea.

I am unaware of any distinction in usage as to whether the obligation comes from someone else or from yourself.

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