Here are some examples from daily life: 1. I should do my homework instead of watching TV. (I am watching TV and I know I am supposed to do my homework) 2. I shall pass this exam (this is the future and it sounds quite formal or "British"; I will pass this exam sounds much better). 3. I have to get a job this month versus I must get a job this month (have to shows external pressure while must shows internal pressure as well) - "must" sounds too strong for most day-to-day usage. If you are in a restaurant, you tell the waiter "I would like to eat this and that" instead of "I must eat this and that" (which sounds rude)

Should is used to make recommendations or give advice and express obligation or expectation. Examples: You should eat more vegetables. You shouldn't smoke. I should leave for work by 9:00. Sara should be here tomorrow. Synoyms would be "ought to" and "be supposed to." Have to is used much like "tener que" in Spanish. Shall is used to indicate future action, but it is rarely used in American English. We prefer to use "will" instead. http://www.englishpage.com/modals/should.html http://www.englishpage.com/modals/haveto.html http://www.englishpage.com/modals/shall.html