Learn Turkish: Dergiyi nerede okuyorsun? - Where are you reading the magazine?

Dergi means "magazine." Because you are reading a specific magazine ("the" magazine), it acts as a definite direct object and needs the accusative case suffix. Since dergi ends in a vowel, we must add the buffer letter y before adding the suffix -i, making it dergiyi.

How do we know to use -i for the accusative suffix instead of -ı, -u, or -ü?

This is based on 4-way vowel harmony. You look at the last vowel in the word dergi, which is i. According to the harmony rules, if the last vowel of a word is e or i, the suffix will also be -i.

Why does the question word nerede come in the middle of the sentence?

In Turkish, the most important piece of information usually goes right before the verb. In question sentences, the question word (like nerede, meaning "where") is almost always placed immediately before the verb to highlight exactly what you are asking for.

Let's break down the verb okuyorsun. Why isn't there an extra vowel before -yor?