Learn Farsi: اگر حرف‌ها خوانا نیستند، تو باید بینایی خودت را چک کنی. - If the letters are not legible, you must check your eyesight.

It uses the exact structure of this lesson: the present stem of خواندن (to read), which is خوان, plus the suffix -ا (-â). This suffix creates an inherent state adjective, literally meaning capable of being read or legible.

Is the word بینایی (eyesight) related to this same grammar rule?

Yes, it is a built-up version of it! First, you take بین (the present stem of دیدن, to see) and add the -ا suffix to get بینا (seeing or sighted). Then, you add the abstract noun suffix -یی to turn that adjective into the noun بینایی (eyesight or vision).

I learned that حرف means word or speech. Why does it mean letters here?

While حرف does mean word or talk in everyday conversation, its literal meaning is letter of the alphabet. When discussing handwriting or legibility (خوانا), it refers to written characters. If you wanted to say words in a written context, you would typically use کلمه‌ها.

حرف‌ها is a non-human plural. Why does it take the plural verb نیستند instead of the singular نیست?

While non-human plurals usually take singular verbs in Farsi, using a plural verb is also correct and is often used to emphasize individuality. Here, نیستند stresses that the individual letters themselves are hard to make out, rather than treating the text as one giant blur.