modals

Express ability in the present

• could

• must

• should

• can

Is the negative form of 'can'

• cannot

• couldn't

• could

• can't

"You can buy tickets online" expresses

• ability

• permission

• possibility

• obligation

Can you close the window? It's cold.

expresses

• permission

• request

• possibility

• obligation

"Can I use your dictionary?"

• permission

• request

• ability

• possibility

No, you can't go to the park until you finish your homework

• is a possibility

• give

  permission

• deny

  permission

• is an obligation

Express strong obligation or duty.

• must

• ought to

• have to

• should

The negative form of must

(a)  

You mustn't talk in the library

• deny

  permission

• is a suggestion

• express

  prohibition

• is a strong suggestion

To give advice

• should

• might

• could

• ought to