Modals: permission

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.

• May is more polite than could, and could is more polite than can.

• We don't usually use a modal to talk about past permission.
✓ I was allowed to wear a knee support during the match.
✘ I could wear a knee support during the match.

• However, we do use could to talk about past permission in reported speech.
✓ The coach said I could wear a knee support during the match.