Modal verbs like should and must are often interchangeably used to express advice, obligation, duty or similar ideas with implied speaker's authority. However, should, unlike must, does not express great confidence that the recommendations will be carried out.
- "Cell phones must be powered down, not on silent or vibrate," the exam proctor said. (policies)
- I will not leave until there is an order from the court saying I must leave. (order)
- In chess, if the king is in check, it must move out of check immediately. (rules)
- If our parents are sick, we should take care of them. (obligation)
- People should eat and drink and enjoy the fruits of their labor, for these are gifts from God. (recommendation)
- You should analyze your most common errors in judgment that you may guard against their recurrence. (advice)
To make written or spoken instructions sound polite, should is used instead of must. Check out these sentences:
- You should arrive at the test venue at 8 a.m. to complete the pre-test requirements. (not must arrive)
- Candidates who wish to transfer to a test date more than three months away should apply for a refund and re-apply for the test. (not must apply)
For predictions, must is not often used, instead should is used to say what people expect to happen (in the future):
- State road contructions should be finished by next week. (not must be)
- The oak seedlings should take less than six weeks to grow about four inches. (not must take)
- The committees should have everything set up, and we should be able to organize ourselves and elect officers day after tomorrow evening.
Meanwhile, must is never used to talk about unfulfilled obligation in the past or recommendations that were not carried out. Should plus past participle of the verb can be used instead:
- You should have been more courteous to Anders. He's the new CEO of the company.
- You should have seen the lengths our staff went to.
- We should have met at the train station. It's closer than the shopping mall.