Modal verbs like must, should, and have to are used to express ideas like obligation, necessity, prohibition, and advice. They do not take suffixes like -s, -ing, or -ed and are followed by the bare infinitive. They come before the subject in questions and are followed by not in negations. Modal verbs do not have distinct tenses and cannot be conjugated like regular verbs. They take a present or perfect bare infinitive depending on whether the action is incomplete or complete. Common uses of modal verbs include expressing obligation with must and have to, absence of necessity with don't have to, and advice with should and ought to.

1. MODAL VERBS
STRUCTURE
1- DO NOT TAKE -S, -ING, OR -ED SUFFIXES
(should, shouldED, shouldING arenot correct)
2- FOLLOWED BYTHE BARE INFINITIVE(INFINITIVEWITHOUTTO)
Ex. He should study more. (correct)
Ex. He should to study more(not correct)
3- COME BEFORETHE SUBJECT INQUESTIONS AND ARE FOLLOWED BYNOTIN
NEGATIONS. Example: MUST I do this? No, you MUSTN’T do it
4- THEY DO NOT HAVETENSES INTHE NORMAL SENSE. THEY CANNOT BE CONJUGATED.
Ex: I will must study more(not correct)
Ex. I will haveto study more(correct). Haveis a semi modal verb and can be
conjugated as a general verb. (semi modals can be auxiliaries and regular verbs).
5- THEY ARE FOLLOWED BYA PRESENT BARE INFINITIVEWHENTHEYREFER TO AN
INCOMPLETEACTIONORSTATE.( * we do not conjugatethe modal verb. Modal verbs
are auxiliaries)
a. We SHOULD STOP the show (Present) *
b. You SHOULD BE STUDYING more(Presentcontinuous) *
6- THEY ARE FOLLOWED BYA PERFECT BARE INFINITIVETO REFERTO A COMPLETE
ACTIONORSTATE. (Usethese structures for pastactions).
a. They SHOULD HAVERUN more today
b. They SHOULD HAVEBEEN STUDYING moretoday.

2. USES ( MODALITY)
OBLIGATION /DUTY / NECESSITY
 MUST expresses DUTYor STRONG OBLIGATION. Ex: I MUSThelp my brother.
 HAVETO expresses STRONG NECESSITY/ OBLIGATION. Ex: I HAVE TO study harder.
 HAD TO is the past formof MUST and HAVE TO. Ex: I HAD TO help my brother to
study math.
 SHOULD/OUGHTTO express DUTY, WEAK OBLIGATION. Ex: they SHOULD recycle
more.
ABSENCEOF NECESSITY
 DON’THAVETO /DON’TNEED TO / NEEDN’T express it IS NOTNECESSARY in Present
and Futuresentences.
Ex: You don’thave to pay for the ticket
Ex: You don’tneed to come to the party tonight
Ex: You needn’t pay for the ticket tonight.
 DIDN’TNEED TO/ DIDN’THAVETO expresses it WAS not necessary in Past Sentences.
We do not know if it was done or not.
Ex: He DIDN’TNEED TO pay for the ticket. ( maybehe paid, maybehe didn’t)
Ex: He didn’t haveto buy her a new dress. ( maybe he did, maybehe didn’t)
 NEEDN’T HAVE+ PASTPARTICIPLE (ModalPerfect)
Used when something was not necessary BUTit was done.
Example: You NEEDN’THAVE BOUGHToranges ( but you did).
PROHIBITION
 MUSTN’T. It is forbidden to do something. Ex: You MUSTN’T smoke here
ADVICE
 SHOULD. Personaladvice. You SHOULD usesunscreen if you go out today.
 OUGTH TO. General advice. You OUGHTTO usesunscreen if you don’t wantto get
burned.