Here's why:
* Modal verbs are helping verbs that are used with another verb to express different shades of meaning (like possibility, obligation, ability, or permission).
* Auxiliary verbs help the main verb in a sentence and cannot stand alone.
In the sentence "I would go to the party," "would" is a modal verb that helps the main verb "go" express a conditional action (something that might happen).
Let me know if you have other grammar questions!