Here's why:
* They express possibility, ability, permission, etc., not actions: Modals like *can, could, will, would, may, might, should, must* describe a speaker's attitude towards an action, not the action itself.
* They are auxiliary verbs: Modals always precede another verb (the main verb). They don't stand alone as verbs.
* They have unique forms for expressing past tense: Instead of adding -ed or changing the vowel, they have their own set of forms. For example, *could* is the past tense of *can*, and *would* is the past tense of *will*.
Here's a table to illustrate:
| Modal Verb | Past Tense |
|---|---|
| Can | Could |
| Will | Would |
| May | Might |
| Shall | Should |
| Must | Had to (sometimes) |
Note: While *must* technically doesn't have a direct past tense form, *had to* is often used to express a past obligation.