Learn Turkish: Ağrılı eklemlerinle yapamayacağın tek hareket çömelmek değil. - Squatting is not the only movement that you cannot do with your painful joints.

It starts with the root verb yap- (to do). Next is the inability suffix -ama- (cannot). Then we have the future participle -yacak, which acts like a relative clause meaning "that will...". Finally, the 2nd person possessive -ın indicates "you" (making the k in -yacak soften to ğ). Literally, it means "the thing that you will be unable to do."

Why use the future participle -yacak in yapamayacağın rather than the present/past -dık?

If the speaker had used yapamadığın (with -dık), it would mean "movements that you currently cannot do" or "have not been able to do." Using -yacak makes it slightly hypothetical or forward-looking: movements that you won't be able to do if you try them. In negative ability contexts, Turkish speakers frequently prefer the future participle to express general impossibility.

Why is the verb çömelmek used in its full infinitive form here?

To say "X is Y" or "X is not Y" where Y is an action, the action needs to be treated as a noun. The infinitive -mek / -mak suffix creates a verbal noun, translating here to "squatting". Because it acts as a noun in this sentence, it can sit right next to değil (is not) to form the predicate: çömelmek değil (is not squatting).

What role is the suffix playing in ?