Which form of verb is used during the use could in a sentence?

The word "could" is a modal verb. It is not a regular verb form itself, but rather a helping verb that combines with another verb to express possibility, ability, permission, or request.

Here's how it works:

* "Could" + base form of the verb: This is the most common usage, expressing possibility, ability, or polite requests.

* Example: "I could go to the store later." (possibility)

* Example: "She could swim when she was five." (ability)

* Example: "Could you please pass the salt?" (polite request)

Therefore, the form of the verb used with "could" is the base form of the verb.