Should is used to give advice, to express an obligation, or to indicate a probability. For example, in
this sentence, the speaker uses should to refer to something he believes that Peter should do; in
other words, to give advice.
Ex: In addition to carrying out all of the tasks included in his job description, Peter fixes any
technological problems that come up in the office and he takes care of all of the scheduling of
appointment. Considering all of the work that he does, he makes too little money and should
request a raise.
Here, should indicates a weak obligation. It indicates an obligation less strongly than must or have
Ex: My husband should go to maternity class with me. It's important for fathers to be involved in their
children's lives, even during pregnancy.
The following uses of should express probability. The weather report indicated that a sunny day was
Ex: The weather report said that it should be sunny this afternoon, but looking at those dark clouds in
the horizon, I think it will storm later. If there's any thunder, they should call off the softball game.
Must expresses obligation or probability. For example, here must indicates an obligation, something
that has to be done. When expressing obligation, it is stronger than should.
Ex: All travellers must fill out the customs forms before they get to immigration, and they must
declare any fruit or vegetables in their luggage.
In this example must indicates a probability. It's highly likely that someone would be tired after
working for sixteen hours straight!
Ex: After her eight-hour shift at the hospital, Angela agreed to cover for her colleague whose son is at
home sick. She must be exhausted after working two eight-hour shifts in a row without any break.
Must not means that something is prohibited as in the following example:
Ex: The children must not drink soda this late in the day, or they will be up all night. Juice or milk
would be healthier for them anyways.
Lastly, have to indicates an obligation:
Ex: Francis has to finish his homework before he can come out and play, because his parents were
upset when his teacher told them that he hasn't been completing his assignments.