Modal Verbs

1. Modal verbs can express obligation, ability, possibility among other conditions, they have to be follow by an other verb in infinity form without 'To'

2. Can

2.1. it used to express possibility and ability

2.1.1. I can speak two languages

2.1.1.1. Ability

2.1.2. It can be dangerous to drive if you are tired

2.1.2.1. possibility

3. Should

3.1. to indicate an obligation or an advice

3.1.1. You should quit smoking if you want to live

3.1.1.1. Advice

3.1.2. He should study if he wants to pass the examen

3.1.2.1. Obligation

4. Must

4.1. to indicate an obligation, a ban and a need

4.1.1. You must brush your teeth three times a day

4.1.1.1. Obligation

4.1.2. You must not drink and drive

4.1.2.1. Ban

4.1.3. You must read this book, it´s fantastic

4.1.3.1. need

5. Could

5.1. it is the past form of CAN as can express possibility and ability

5.1.1. I could speak english when I was young

5.1.1.1. ability

5.1.2. It could have been worse

5.1.2.1. possibility

6. Would

6.1. to declare a preference and to ask question politely

6.1.1. I would like to go to New York someday

6.1.1.1. Preference

6.1.2. Would you like some coffee?

6.1.2.1. Question politely