Referring to the other answer, I don't know if classifying them as strong or weak really applies. I would say each of these words expresses how imperative it is to do something. Other words used in the sentence can be at play. For example, let's say you're suggesting to someone about how important it is to see a doctor. If you say, "You should see a doctor," it's expressing a real need to go to one. Not a huge need, but a need none the less. However, if you word it "Maybe you should see a doctor," this changes things. It's a suggestion now. You can't really say "Maybe you must see a doctor." This is a contradiction of terms because "must" implies that it's imperative to do something. To make it very imperative, you can say "You must see a doctor immediately!" You see how the other words at use can change the meaning?

The difference lies in their strength. "Should" is the weakest. It implies that it's good to do something. It's good for suggestions. Example: "You should repair your bike. It would work better" "Must" is stronger. It means it's important for you to do something (obligation). Example: "You must repair your bike. It looks unstable and you might hurt yourself" "Have to" is the strongest. It means it's very important, or you were ordered by other people, to do something. Example: "You have to repair your bike. You can't ride on it until it is repaired. I forbid you". In practice it usually doesn't matter that much if you mix up these words, so don't worry. Hope it helps. Maybe other people will add something to it. Good luck. :)

Short answer: YES. If you confuse them you will sound like a poor speaker. We will notice. "Must" and "have to" are obligations. "Should" is advice. There is PLENTY of advice online, plus exercises, for these modals. Here are a few for starters: www.englishclub.com/grammar/verbs-modals-have-to-must-not-quiz.htm www.grammar-quizzes.com/modal3.html www.esl-classroom.com/grammar/shouldandmust1.html Have fun! :)