"Should" is most commonly used to make recommendations or give advice. It can also be used to express obligation as well as expectation.
Most modal verbs behave quite irregularly in the past and the future. Study the chart below to learn how "should" behaves in different contexts.
2. Frank should have eaten low-fat foods. That might have prevented his heart attack.
3. You really should start eating better.
2. Sarah shouldn't have smoked so much. That's what caused her health problems.
3. Sarah shouldn't smoke when she visits Martha next week. Martha hates when people smoke in her house.
We should return the video before the video rental store closes.
"Should" can also express something between recommendation and obligation. "Be supposed to" expresses a similar idea and can easily be used in the past or in negative forms.
2. Susan should have arrived in New York last week. Let's call her and see what she is up to.
3. Susan should be in New York by next week. Her new job starts on Monday.
2. Susan shouldn't have arrived in New York until yesterday.
3. Susan shouldn't arrive in New York until next week.