Modal and semi-modal verbs are used to express following functions:
Permission or request: can, could and may, e.g., May I go to the loo?
Ability: can, could, be able to and be capable of, e.g., I could rollerskate when I was 12.
Obligation: must and have to, e.g., You must listen to me.
Prohibition: mustn't and can't, e.g., You can't smoke indoors.
Lack of necessity or obligation: don't have to, didn't have to, needn't, didn't need to, and don't need to, e.g., You don't need to wear a uniform in college.
Advice: should, ought to, must, need to and could, e.g., If you have a headache, you should take an aspirin.
Possibility and probability or speculation or deduction: can, may. might, could, must, can't and mustn't, e.g., Nobody's at home, so Susan must be out.
Necessity: must, need to, and have to, e.g., You need to save some money if you want to buy a car.
Toy story introductory video
Past modals
1.- Simple modals: could, was/were able to, had to, didn't need to and didn't have to., e.g.,We could run fast when we were younger.
2.- Perfect modal: might have + Past Participle, could
have + Past Participle, can't have + Past Participle, should have + Past Participle, ought to have + Past Participle, need have + Past Participle and needn't have + Past Participle, e.g.,You should have studied for the final exam.
Past Modals explanation video
Past Modals
1.- Simple modals in past.
1.1.- could.- Past abilities: I could rollerskate when I was twelve.
1.2.- would.- Future in the past.- Before the pandemic, I would be at school at this time.
1.3.- had to.- Past obligations: I had to stop using prints.