Grammar

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Modals express abstract meanings. Should and ought to are used to give advice or to recommend something. Must and have to are stronger; they indicate an obligation or necessity. Modal auxiliaries, with the exception of have to, are never conjugated. They are always followed by a main verb in the base form (without to). Modals do not require another auxiliary to form questions and negatives.

Should expresses advice or recommendation. You should always iron your uniform, so you look your best.
Ought to is a less frequently used synonym of should. You ought to get some sleep; you look tired.
Should and ought to can be followed by the present perfect to express advice about a past moment. They should have saved their money; they wouldn’t be broke now. He ought to have hurried so he wouldn’t have missed his flight.
In affirmative sentences, must and have to both express obligation.

You must respect the company’s rules and regulations.

You have to pay for public transit…

In negative sentences, must and have to express opposite meanings:

  • must not expresses negative obligation;
  • not have to expresses absence of obligation.

You mustn’t smoke in public buildings. It is forbidden by law.

You don’t have to eat meat if you don’t want to; we also serve vegetarian options.
Only have to can refer to the past.

Yesterday, I had to* call an important client.

Yesterday, I must call an important client.

Making recommendations – Grammar