Modals: obligation and necessity

Form

• All modals (will, would,shall, should, can, could, may, might, must) and the semi-modal ought to have only one form.
• Modals are followed by the bare infinitive (simple or continuous) or the bare perfect infinitive
  eg To by should be very fit by now.
  To by should have recovered by now.
• The semi-modals have to and need to change their form depending on person and tense
  eg The doctor said I had/needed to give up red meat.

• There is usually no difference in meaning between must and have to. However, we are sometimes more likely to use must for personal obligation (making our own decision about what we must do) and have to for external obligation (someone else making a decision about what we must do).

• We can also use will have/need to to express future obligation.
✓ You'll have/need to be more careful about what you eat in future.

• It is unusual to use must for questions.We usually use have/need to.
✓ Do I have/need to take this medicine before every meal?

• Must cannot be used as an infinitive. Use to have to.
✓ I'd hate to have to have injections every day.
✘ I'd hate to must have injection everyday.

• Mustn't and don't/doesn't have/need to have different meanings.
✓ You mustn't do that! (Don't do that!)
✓ You don't have/need to do that. (You can do that if you want to but it's not necessary.)

• Needn't ( + perfect infinitive) always refers to an action that happened.
Didn't have to and didn't need to can refer to actions that did or didn't happen.
✓ I needn't have gone to the doctor. (I went but it wasn't necessary.)
✓ I didn't have/need to go to the doctor because I suddenly felt better. (I didn't go.)
✓ I didn't have/need to go to the doctor but I went just to be on the safe side. (I did go.)

• Be careful with the verb need. It can also take the -ing form.
✓ I need to sterilise this syringe.
✓ This syringe needs sterilising.