Learn Turkish: Arabanın aküsü kesin bozulmuştur. - The car's battery has definitely broken down.

While -DIr usually shows assumption or probability, it can also show strong deduction. When paired with an adverb like kesin (definitely), the -mIştIr ending expresses a highly confident conclusion about the past. It effectively means I didn't see it happen, but I am entirely sure it did.

Why do we use the -mIş past tense here instead of the regular -DI past tense?

Because the speaker didn't personally witness the battery breaking down. They are making a deduction based on the current situation (such as the car refusing to start). Using -mIştIr instead of the direct past tense shows this is an inferred conclusion rather than an observed event.

Why is the word for battery written as aküsü instead of just akü?

It is the second part of a possessive noun compound. Arabanın means the car's (using the genitive suffix -nın), so akü (battery) must take the third-person possessive suffix -sü to complete the link, forming arabanın aküsü (the car's battery).

Is bozulmuştur a passive verb?

Yes. The base verb is bozmak (to break or ruin). By adding the passive suffix -ul, it becomes bozulmak, which is commonly used to mean to break down for machines or to go bad for food.