I saw an answer in a old thread that I do not understand. I made a comment yesterday asking the user to explain a particular step. But I realize now that the thread is old and the user is no longer active on MSE. Am I allowed to post a new question, with a link to that particular answer and seek help from others in understanding it?
Linking back to the old answer (not just the question; the URL for the answer itself can be obtained by clicking the share tool between the answer and the answerer's profile name) provides context, and helps those who will explain it.
You should also quote the relevant section of the old answer, and describe which parts of it don't make sense, and why not.
Your new question should then appear as a linked question on the old one (a stronger association than being a related question).