şeker is general sugar But when it becomes şekeri, then its a 'the', specific sugar If an extra vowel is added to the end of a noun, when it is not supposed to be there, then the noun is specific, not general. For example, the vowel in 'su' is part of the word and it means water in general. When you say suyu, you added a vowel to the end of the word, so it becomes 'the water'
Accusative case = object in a sentence. In a sentence, you speak English, English is the object. And in Turkish, it becomes a specific object, the something, by adding accusative marker. BTW, there are many aspects in common between Turkish and japanese, but this phenomenon isn't the case in Japanese. If you specify an object, you add similar words to bu, şu, o in Turkish. Say, Şu balığı yerim. I eat that fish = I eat the specific fish = I eat the fish.
"Evet, şekeri yer." Translation: Yes, he eats the sugar.
let's look at the question and break it down, literally.
"Evet, şekeri yer." Yes - Evet.
şekeri - The sugar. şeker - Sugar.
Yer - eats. Turkish follows the word order: Subject-Object-Verb.
The subject being yes then a comma. What, is being said "yes" to? The object being, şekeri, the sugar.
Yer - eats, is the verb. Only a He/She & it can eat. I have tried to explain this the best I can.
Not only subjects but all nouns/pronouns are often omitted in Japanese as well. But Turkish offers more clues through suffixes than Japanese, such as yer at least tells you who eats, in this case she or he or it. In Japanese, to eat can mean I eat, you eat, he eats, she eats, it eats, we eat, you guys eat and they eat. So figure it out!