Learn Turkish: Yapımcı farklı bir kadro seçebilirdi. - The producer could have chosen a different cast.

It combines the verb root seç (choose) with the ability suffix -ebil, the aorist suffix -ir, and the past tense suffix -di. Using the aorist past tense with the ability suffix is the standard way to express a counterfactual past action: something the producer had the ability to do, but ultimately didn't.

Could seçebilirdi also mean "used to be able to choose"?

Yes! Grammatically, the ability aorist past can express either a counterfactual event ("could have chosen") or a past habit or ability ("used to be able to choose"). Context tells us which one it is. Here, casting a specific project is a one-time event, so "could have chosen" makes the most sense.

Why is the word order farklı bir kadro instead of bir farklı kadro?

In Turkish, adjectives generally must be placed before the indefinite article bir. If you say bir farklı kadro, it sounds unnatural to native speakers. You will almost always see the pattern as Adjective + bir + Noun.

How is the word yapımcı (producer) formed?

It is built from the verb yapmak (to make). The suffix -ım creates the noun yapım (a production), and the agent suffix -cı turns it into a profession, literally meaning "the production person" or producer.