Questions & Answers about Gazetede bir ilan gördüm.
Why doesn't ilan have the accusative suffix? Shouldn't it be ilanı gördüm since it's the object of the verb?
We only use the accusative suffix (like ilanı) when the direct object is specific, usually translating to "the" in English. Because you saw "an advertisement" (bir ilan), it is an indefinite object. Indefinite objects stay in their plain dictionary form and usually sit right next to the verb.
Why does Turkish use the suffix -de for "in the newspaper" instead of a separate word for "inside"?
The locative suffix -de / -da is used to show where an action takes place, covering "in", "on", and "at". You only need a separate word for "inside" (like içinde) if you want to emphasize something being physically enclosed or stuffed between the pages. For reading something printed in a publication, gazetede is the standard choice.
Could I change the word order to Bir ilan gazetede gördüm?
You could, but it changes the focus of the sentence! In Turkish, the most important information usually goes right before the verb. The current sentence Gazetede bir ilan gördüm emphasizes what you saw (bir ilan). If you move gazetede right next to the verb, you are stressing where you saw it (in the newspaper, as opposed to on the internet or on TV).