-Modal verbs are invariable: one form for all the persons. That means you mustn't add -s to the third singular person. You must study. She musts study.
-Modal verbs don't need the "do support" in negative and interrogative sentences. You don't should tell him. You shouldn't tell him. Do you can swim? Can you swim?
-Modal verbs are always followed by the base form of the verb (infinitive without TO) You must to go. You shouldn't eating so much.
-Modal verbs do not have either the infinitive form, the past participle, the future tense, or the -ing form
WARNING All these rules must be applied to modal verbs. But what about the verbs HAVE TO BE ABLE TO and NEED TO ?
HAVE TO and BE ABLE TO do not share all the features of the modals but they do share some of the uses: obligation= ability= HAVE TO BE ABLE TO
Let's see some examples... I don't have to wake up early tomorrow. It's Sunday. You will have to study hard if you want to pass the exam. She has to go now. Her mum is waiting for her. Do you have to work at the weekend?
more examples... After Bachillerato, you will be able to prepare your exam of selectividad Were you able to pass the driving exam?
NEED TO = NECESITAR is not a modal verb since it has the same forms as the so-called full verbs (go, change, drive, drink,etc.)
POLITE REQUEST COULD - Could you help me with this exercise? It’s so difficult. COULD is also used to make a polite suggestion. A: I’m going to Holland next week. B: Oh, really? You could visit your Dutch relatives then. MAY - Mrs. Lola, may I go to the toilet?
REQUEST & co. WILL YOU? (ask someone to help you) - Will you help me with the suitcases, please? They’re so heavy. WOULD (formal request) - Would you open the window, please? It’s so hot in here. SHALL I? (offer to help someone) - Shall I help you with your luggage? WOULD (make an offer) - Would you like a sandwich?
NECESSITY/OBLIGATION MUST (strong obligation: law, authority)
Students in private schools must wear a uniform. (RULE)
All of our friends are going to that party. We must go too. (NECESSITY)
NECESSITY/OBLIGATION MUST HAVE TO The obligation comes from the speaker The obligation doesn’t come from the speaker MUM: You must be home by 11 o’clock. You go out. When it’s time to go, you can say: “ I’ll leave. I have to stay home by 11 o’clock.”
MUST - HAVE TO They have more or less the same meaning but… HAVE TO is used for all the forms MUST doesn’t have. Moreover, HAVE TO is used in the interrogative sentence.
Do you have to go right now?
Yesterday I had to do the washing because the washing-machine broke down.
If you want to be successful, you will have to work very hard.
NEGATIVE FORM MUSTN'T DON'T/DOESN'T HAVE TO Prohibition No obligation, no necessity
You mustn’t smoke in the premises. (IT IS PROHIBITED, IT IS NOT ALLOWED)
You don’t have to wear a uniform at school. (IT IS NOT NECESSARY)
POSSIBILITY CAN Scissors can be dangerous if not used properly. (+++possible) MAY (++possible) I may go to the party. (I’m not sure, it’s possible) MIGHT (+possible) I might go to the party. (I’m not sure, it’s improbable)