Learn French: Tu les paies avec un chèque. - You pay them with a check.

payer (to pay) is a stem-changing verb ending in -yer. For most subjects (je, tu, il/elle, ils/elles), the y changes to an i in the present tense, making it paies for tu. (Note: For payer specifically, keeping the 'y' as payes is also allowed, but changing it to i is much more common and follows the exact rule for verbs like envoyer!)

Why does les come before the verb instead of after it?

In French, direct object pronouns like les (them) are placed directly before the verb. While in English we say 'You pay them', in French it translates literally as 'You them pay' (Tu les paies).

Can les refer to both people and things here?

Yes! les is the plural direct object pronoun for both masculine and feminine nouns, whether they are people or things. So this sentence could mean you are paying people (like contractors) or paying things (like bills).

I usually say 'pay by check' in English. Why is it avec un chèque here?

In French, you can say either avec un chèque (literally 'with a check') or par chèque ('by check'). Notice that if you use par, you drop the word un. Both are very common ways to say how you are paying.