Learn Farsi: پرستار در کار خود خیلی توانا است.

It is built from the present stem of the verb توانستن (to be able to), which is توان. By adding the suffix -ا (-â) to a present stem, you create an adjective that shows an inherent state or permanent ability. So, توان (can) + -ا = توانا (capable).

Why does the sentence use خود instead of a possessive ending like کارش to mean "her work"?

While کارش (her work) is perfectly fine and common in everyday Farsi, using the reflexive pronoun خود (self/own) refers back directly to the subject of the sentence (the nurse). Here, کار خود translates nicely to "her own work" and sounds a bit more professional and precise than just using the clitic -ش.

Does the word پرستار (nurse) also use a present stem to form its meaning?

Yes! It uses the present stem of پرستیدن (to worship or care for), which is پرست. But instead of the -ا suffix, it takes the agent suffix -ار (-âr) to describe the person doing the action. So, a پرستار is literally a caretaker.

Could I use a word like قوی instead of توانا to say the nurse is capable?

Not quite in this context. While both can sometimes translate to "strong", قوی usually refers to physical strength. توانا focuses on being competent, skilled, or having the inherent capability to do a job well, which makes it the natural choice for describing professional ability.