Learn Turkish: Geri dönüşüm yaptıkları için doğayı koruyacaklar. - They will protect nature because they recycle.

The verb root is yap (to do). We add the special suffix -tık, which turns the verb into a noun-like word (the doing of). Then we add the third-person plural possessive -ları (their) because the subject is "they". Finally, adding için (for/because) gives us literally "for their doing"—which translates to "because they do".

The -tık suffix looks like a past tense ending. Does yaptıkları için mean "because they recycled" or "because they recycle"?

It can actually mean both! The -dIk (or -tık here) suffix is used to build "because" clauses for both past and present actions. You have to rely on context to know which one it is. Since the main action here (koruyacaklar) is in the future, translating it as a general present habit ("because they recycle") makes the most sense. If it were a future action ("because they will recycle"), it would use -acak instead: yapacakları için.

Why is the word for nature written as doğayı instead of just doğa?

Doğa (nature) is the specific object receiving the action of the verb korumak (to protect), so it needs the accusative case ending. Since doğa ends in a vowel, we add the buffer letter -y- followed by the suffix -ı, giving us doğayı.

Why is there no word for "they" in the Turkish sentence?