Hello Alexander! This will give you an idea of the meanings of these words: "need" is a verb expressing necessity or obligation. For example: "I need help now." "must" means to be obliged to; should (expressing necessity). For example: "You must show your ID card" "should" is used to indicate obligation, duty, or correctness, typically when criticizing someone's actions. For example: "He should have been careful, but he wasn't and now he is paying the price." Let me know if that helps!

there are many uses for these words and this would take learning them. You have to ask a teacher to give you a lesson or two