should vs. have to vs. must in specific context?

Question

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.

Answer 1

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.