Learn French: Il ne répond pas aux messages. - He does not reply to the messages.

The verb répondre (to reply/answer) is a regular -re verb. To conjugate it in the present tense for il (he), elle (she), or on (we/one), you drop the -re ending to get the stem (répond), and you don't add any extra letters. That is why it stays as il répond. For je and tu, you would add an s (je réponds, tu réponds).

Where does the word aux come from?

Aux means "to the" for plural nouns. The verb répondre is followed by the preposition à (to). When à is placed right next to the plural word les (the), they combine into the single word aux. You can never say à les in French!

How do you say "does not" in this sentence?

French doesn't have a word for the helping verb "does." Instead, it uses the two-part negative frame ne ... pas around the conjugated verb. To make il répond negative, you just put ne before the verb and pas after it, creating il ne répond pas.

Can I just say "Il ne répond pas les messages"?

No, you need the preposition à (which turns into aux here). Just like in English where you "reply to" something rather than "reply" it, French requires répondre à.