You're right, but there's a little more to "must" and "should" than you mentioned. If you must do something, you have no choice but to do it. "I must..." and "I have to..." both mean the same thing, but "must" and "have to" act different grammatically. "Must" is an auxiliary verb and "to have to" is not; so you'd say "I must not..." and "I don't have to..." for negation, and "Must I...?" and "Do I have to...?" for questions. Their negated forms also have different meanings. "I must not..." means that I have no choice but to not do something, whereas "I don't have to..." means that I can choose whether to do something or not (e.g. "You must not commit murder!" or "You don't have to do it, but you can if you want."). "Must" also only exists in the present tense, so you have to use "to have to" in any other tense. Note that the pronunciation of "have" is different in "to have to" than in "to have". The "v" in "to have to" is pronounced with an "f" sound. It's similar to how the "d" in "used" is pronounced as a "t" in "I used to..." or "I am used to,,,", but it's pronounced with a normal "d" sound when it's just the past tense of "to use". "Should" means that you don't have to do something, but you have the choice not to do so. If you said "You should become a lawyer", you would be saying that becoming a lawyer is a good idea, but not compulsory.

You've already got it :) You can think of COULD / SHOULD / MUST as increasing levels of urgency. For example: "I could go to the shops but I'm too lazy" "I should go to the shops because we've almost run out of milk" "Must" and "Have to" are basically interchangeable, but only work for present and future: "I must go to the shops [now] otherwise my mum will kill me!" "I have to go to the shops [now] otherwise my mum will kill me!" or "I can't meet you tomorrow because I must to go to the shops" "I can't meet you tomorrow because I have go to the shops" (or: ....because I have to go shopping) For the past tense you would use "had to" (no "must" equivalent): "I couldn't meet you yesterday because I had to go to the shops" [I had to --> I couldn't avoid it] Hope that helps