Must and have to = You don't have a choice Should = you don't have to but it would probably be a good idea to do it.

We use should to suggest something. "It is advisable to..." The modal should expresses opinion, one person's point of view. We should protect our environment. It makes good sense. We use must to express a stronger point of view. "We need to..." "We have to..." The modal must also expresses opinion, one person's point of view. We must protect our environment, or our resources will disappear. We can also use 'have to' to express a strong obligation. When we use 'have to' this usually means that some external circumstance makes the obligation necessary. I have to arrive at work at 9 sharp. My boss is very strict.

'Must' and 'have to' are very close and many times can be interchanged. 'Must' implies something about which you are absolutely certain or something for which there is a strong obligation for personal reasons. "That wind must be fifty kilometers per hour." "I simply must stop smoking." 'Have to' is used more when there is a strong obligation caused by external circumstances. "I have to get there by five or my wife will kill me!" There is a very big difference in the negatives, though. "You must not go in there again." - Under no circumstances go in there again. "You don't have to go in there again." - You can go in there again, but is isn't necessary. A quite different meaning. Finally, 'should'. If you 'should' do something, it is recommended, but you are not forced to do it. If you 'must' or 'have to' do something, you are obligated to do it.