We have some canonical Q-As (listed in ¿Qué preguntas canónicas tenemos? What are the canonical answers we've discovered over the years?), but we need more, so that we can point learners to some reliable information every time certain often-asked questions come up, such as subjunctive. Let's collect in one place a list of the top priority topics that we really ought to have a canonical answer for. I propose that we do it in a community wiki answer here.
(To come up with a canonical q-a, sometimes it's helpful to start with an existing Q-A and perhaps edit it a bit, sometimes it's better to start from scratch.)
This question came up when I read a recent question about the subjunctive from someone who knows stackexchange inside and out, but it looked as though they had apparently not read the basics about the subjunctive that have already been covered here. Then I realized that it is not easy to find the basics that have already been covered about the subjunctive here!