"The woman eats salt."
Translation:Kadın tuz yer.
The former is in the nominative case and is used for subjects and general direct objects (meaning you would just say "salt" as a direct object). Tuzu is in the accusative case and is for specific direct objects (meaning you would have to say "the salt" as a direct object). Tuzu will never show up as a subject if it is in the accusative case.
p.s. tuzu could have a different suffix on it as well that you will learn later :)