Learn Hardcore Turkish: Yağ yemeklere lezzet katar.

Because yağ is the subject of the sentence. In Turkish, the subject normally appears in the nominative (no case ending). The verb katar agrees with this nominative subject.

Why doesn’t lezzet take the accusative suffix -i if it’s the direct object?

In Turkish, a direct object only takes the accusative -i when it’s definite or specific. Here lezzet (“flavor”) is indefinite (we’re talking about flavor in general), so it stays in the bare nominative form even though it’s the object of katmak.

What do the endings on yemeklere mean?

yemek = “food” or “dish”

-ler = plural, so yemekler = “foods” or “dishes”
-e = dative case, “to” or “for”
Together yemeklere means “to (all) foods” or “to (various) dishes.”

What tense or aspect is katar, and why is it used here?

katar is the Turkish aorist (geniş zaman) form of katmak (“to add”). The aorist in Turkish often expresses general truths, habitual actions, or universal facts. Since the sentence states a general fact—“Oil adds flavor to foods”—the aorist is the appropriate choice.