Question

should表示按常理的推测.can表示理论上或习惯上的可能性.

Answer 1

should 指说话人的期望推中的可能性,如： They should have arrived in Beijing by this time. 他们这个时候该到北京了.They should arrive at noon. 她应该在中午到达. must 指根据逻辑推断出来的可能性,表示一种必然性,如： If the lights were on, they must have been at home. 如果灯亮着,那么他们一定在家.You must be very tired. 你一定很累了. may 指具有一定量度的可能性,即可能属实但不确定,表示一种或然性.如： There may be some rain today. 今天可能会下点雨.I may be back next year. 我可能明年会回来. can 表示介乎于 must（必然性）和 may（或然性）之间的可能性.如： Such things can and do happen. 这样的事情可能而且确实发生了.long-term therapy can last five years or more. 长期治疗有可能持续5年乃至更长时间.