Thelma

Modals are special verbs which behave very irregularly in English. Modal verbs are special verbs which behave very differently from normal verbs.

For the purposes of this tutorial, we haveincluded some expressions which are not modal verbs including had better, have to, and have got to. These expressions are closely related to modals in meaning and are often interchanged with them.Examples:
He will can go with us. Not Correct
She musted study very hard. Not Correct

Examples:
He should not be late.
They might not come to the party.

Examples:
He can  speak Chinese.
Sheshould  be here by 9:00.

3. Many modal verbs cannot be used in the past tenses or the future tenses.

2. You use "not" to make modal verbs negative, even in Simple Present and Simple Past.

1.Modal verbs do not take "-s" in the third person.

Diferencies

May and Might

May and Might manners are sinónomos and indicate an action that can happen in the future.

Must

The must-modalindicates obligation.

Should and Ought (to)

Should manners and ought to indicate why the requirement is not true. Should and ought are synonymous.

Would

The modal Would followed by the wordlike is a polite way of indicating a preference
Shall and Will

Manners will / shall "+ main verb form the future tense and indicate an intention or an action will happen in the future.

CouldCould modal indicates a possibility or ability in the past.

Can

The modal can, like the Spanish verb power, indicates a potential or ability.

Modals verbs are a class of auxiliary verbs.Modal verbs are also called modal auxiliaries or simply manners. There are ten English modal verbs:
Could, May / Might, Shall Should, Would will, Must ought to.

Example
Tom can help you.
I couldhave told you that.
I will / shall close the door for you.
I may / might go to the park, or I may / might stay home.
Tom must see a doctor immediately.
I should / ought to go home now.
I would...