should / ought to — Should Do

should and ought to express advice or a mild obligation, meaning “should do ~.” In junior high school, should is the main form, while ought to is studied as a supplementary form.

Basic Rules

Use should + the base form of a verb to express “should do ~” or “it would be better to do ~.” It is weaker than must and is natural when giving someone advice. The negative is should not (shouldn't), meaning “should not do ~,” and the question form is Should I ~?, meaning “Should I do ~?”
ought to + the base form of a verb has almost the same meaning: “should do ~.” It is slightly more formal than should, but in modern English should is used far more often. had better + the base form of a verb gives stronger advice, meaning “you had better do ~” or “it would be bad if you did not,” with a warning-like nuance.

Points to Note

① Difference in strength between should and must: must means “you absolutely have to do it,” while should means “it would be better if you did it.” Using should for advice sounds milder and kinder.
② had better gives strong advice, but when used with someone senior, it can sound like an order meaning “you had better do it, or there will be trouble.” It is natural with friends or children, but when giving polite advice, should or Why don't you ~? is safer.
③ should have + past participle expresses regret: “You should have called me.” This is studied from the third year of junior high school through high school.
④ Suggestions and offers: Shall I ~? and Shall we ~? are also important uses of modal verbs.

Practice

1. Write “You should exercise more” in English.
2. Write “Should I meet him today?” in English.
3. Write “You had better not swim here” using had better as strong advice.