You ___ not smoke in your car especially if there are children sitting in the back (must/should)

Question

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Answer 1

Both "must" and "should" work in this case, but the meaning is slightly different.

Must implies a demand or requirement.

Should implies a suggestion or a rule or generally agreed upon ethics within society. Typically, if you "should" do something, I might have a follow up statement that backs this claim up with evidence.

I would personally say "should" is more appropriate in this context.

Answer 2

Both "must" and "should" would be grammatically correct. The difference is in the severity. To say that someone "should not" do something is to advise against it. To say that someone "must not" do something is akin to saying "I expressly forbid you from doing it — it is completely out of the question." However, "must not" does not necessarily imply that the speaker actually has any authority over the person to whom they are speaking or the ability to punish them if they do what they "must not" do anyway.