Should / Ought to

Should is a modal verb used to give advice and recommendation.
- You should see a doctor.
- You should keep your promises.
- Should we buy her a present?

Should can also be used for obligation. For example:
- I should call my Mum tomorrow.
- He should apologize to Mary.

Should is also used for when something is expected.
- Tony should arrive here at 12.00.
- It's 12.30, and Tony should be here.
- This book is wrong, tha answer should be "A".

Note that have to inflects like a normal verb, it can have different tenses and uses auxiliary verbs for questions and negatives. For example:
- Do we have to go out tonight?
- You don't have to come with us.

Must doesn't use auxiliary verbs or different tenses.
- Must we go out tonight? (This sounds rather British, I really don't want to go out tonight.)
- We mustn't forget the tickets. (No auxiliary verb "to do", just add not.)

Ought to

Ought to can be used instead of should. It is less common than should and sounds rather British.
- You ought to say thankyou for the present. (You should say thankyou for the present)

Negatives and questions are less common and even more British.
- We ought not to be late for the meeting.
Questions are often made with "Do you think.."
- Do you think I ought to get up earlier?

Questions with just ought to are rare.
- Ought I to get up earlier?

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