Learn Turkish: Aynı reklamı on beş dakikada bir tekrarlıyorlar. - They repeat the same commercial once every fifteen minutes.

It uses the standard Turkish formula for regular intervals: [Time Period] + Locative Case (-da/-de) + bir. Literally, it translates to "one in fifteen minutes". You can use this pattern for any time period, such as iki saatte bir (every two hours) or ayda bir (once a month).

Why does "reklam" take the accusative suffix "-ı" here?

Reklam (commercial) is the direct object receiving the action of the verb tekrarlıyorlar (they are repeating). Because it is modified by the word aynı (same), it points to a specific, known commercial, which requires the specific accusative case suffix -ı.

How is the verb "tekrarlıyorlar" formed from the noun "tekrar"?

It starts with the noun tekrar (repetition). The suffix -la turns it into the verb stem tekrarla- (to repeat). When we add the present continuous suffix -ıyor, the final "a" in the stem narrows to an "ı" (a process called vowel narrowing), giving us tekrarlıyor. Finally, the plural suffix -lar is added for "they".

Can I change the word order and put "on beş dakikada bir" at the beginning of the sentence?

Yes! Turkish word order is flexible, but the position of a word changes the emphasis. The most emphasized information usually comes right before the verb. Keeping on beş dakikada bir before the verb emphasizes the frequency of the repetition. Moving it to the start of the sentence would shift the focus slightly to the commercial itself.