Learn Turkish: Yaşlı çiftçi buğday satmaz. - The old farmer does not sell wheat.

This sentence uses the negative form of the aorist tense, which describes general habits or facts. Instead of satmıyor (is not selling right now), we use -maz for the "he/she/it" form to mean he "does not sell" as a rule. Since satmak (to sell) has an "a" vowel, it takes -maz instead of -mez.

Why isn't buğday (wheat) in the accusative case here?

Because the farmer isn't selling a specific batch of wheat ("the wheat"), but rather wheat in general. When a direct object is general or indefinite in Turkish, it doesn't take the accusative ending and must sit directly next to the verb.

Can I use eski instead of yaşlı to mean "old"?

Not for a person! In Turkish, eski is used for objects that are old or worn out, or for former titles. For living things like people or animals, you must use yaşlı, which literally means "with age" (from yaş, meaning age).

Does çiftçi (farmer) come from another word?

Yes, it comes from çift, which means "pair" (historically referring to a pair of oxen used for plowing). The suffix -çi turns it into a profession, so a çiftçi is someone who farms. If you want to say "farm," the word is çiftlik.