Here's why:
* They express ability, permission, possibility, or request. They don't describe an action directly but modify the main verb that follows them.
* "I can swim." (ability)
* "You could borrow my car." (permission)
* "It could rain tomorrow." (possibility)
* "Could you pass the salt?" (request)
* They don't change form for different persons or tenses. You don't say "I cans," "he caned," or "she coulding."
* They always precede the main verb. They are never used alone.
Let me know if you'd like to know more about modal verbs!