"She has to wear glasses, because her eyes are weak."
Translation:Ŝi devas porti okulvitrojn, ĉar ŝiaj okuloj estas malfortaj.
I felt the explanation from Lernu was excellent for "si." However, when I came back to the example, I would interpret the sentence to be that she has to wear glasses because another woman's eyes are weak. The subject is neither the speaker nor the person being spoken to. Therefore, the second use of a pronoun must be a form of "si" so as to refer to the original subject... At least that's how I read it. My interpretation does not match the answer so I have a gap in my understanding. Any suggestions?
Each sub-clause has to be considered separately. See the "Subclause" subsection of the "Si" section at https://lernu.net/en/gramatiko/pronomoj#si
The predicate of a subclause has its own subject. If you use si(a) in a subclause it always represent the subject of the subclause, never the subject of the main clause[.]
Si can not itself be the subject. It is also not possible to use si as the subject of a subclause,or part of the subject of a subclause with the aim of making si represent the subject of the main clause.
I got my translation wrong, because I used "bezonas" instead of "devas". After looking up the difference between the two, this is my mnemonic for keeping them straight: "bezoNi" is followed by a Noun, and "deVi" is followed by a Verb. So, for example, "Mi bezonas la libron; mi devas legi la libron."