Clarifying the use of the subjunctive mood in Spanish
First of all: understanding the general meaning of subjunctive forms is not enough to be able to use it or even make sense of every use case. It's a good beginning but you still need to pay attention to the different contexts in which they are used because there are a lot of idiomatic cases that you just need to get used to. This happens with most language structures. As an example, consider a "simple" case: the English past forms. Oh, good, I get them, they are used when talking about something in the past. However you get:
I wish she came (???? Why past?)
I knew you were here (???? in Spanish we also use such as "tense sequence" here but many languages would use the present tense because you're still there, right?). [https://en.wikipedia.org/wiki/Sequence_of_tenses]
If you did [???, this is not past] that, I'd be happy
You get a lot more of that in more complicated cases, such as the Romance subjunctive verb forms.
Now for your examples:
1) That's probably the more difficult one: in relative clauses (the one that, those who, the car that...) both indicative and subjunctive forms are used, with a change in meaning. If you know for sure that there are in fact members learning the same language as you and you have even met them, so you have a "mental picture" of them, then you'd use the indicative. That is:
Puedes hablar con otros miembros que _están_ aprendiendo tu idioma.
Would entail almost a guarantee that you'll find those members and even that I know who they are
Puedes hablar con otros miembros que _estén_ aprendiendo tu idioma.
means that such members may or may not exist and that, in any case, I haven't met them personally.
Another example, the normal way of saying such things as
I want to talk with someone who speaks Japanese.
I need to find a shop that sells socks.
would use subjunctive:
Quiero conversar con alguien que _hable_ japonés
Necesito encontrar un tienda que _venda_ calcetines.
Because any person or any shop that fulfils such a condition will do. I haven't met that person or seen that shop yet.
If I use indicative:
Quiero conversar con alguien que _habla_ japonés.
Necesito encontrar una tienda que _vende_ calcetines
I mean that I know such a person, I'm looking for a concrete person who speaks Japanese and whom I've met or a particular shop that I already know.
That is, if I say the first version (with subjunctive) and someone says: "Ok, here is one person who speaks", I'll say "great, what I was looking for!". In the second case it may be the case that I'd say "Oh, yes, but I was looking for someone other Japanese speakers, not this one".
2) This one's easy: after "para que" (so that, in order that,...) only subjunctive forms are possible. The rationale here is that if you do something in order to achieve a result, such a result is far from sure, at least at the moment in which the main action [in the example, reposar el féretro] is performed.
3) Same thing here: "antes [de] que" demands subjunctive. The reason here is similar to the the previous one, if you do something before something else happens, at that point in time when the main action is fulfilled, you can't be sure whether the next thing will in fact end up happening.
Those two last examples are very typical: there is usually a reason for the subjunctive from the native speakers' perspective but you, as a learner, would be better off acknowledging that this is the proper way of speaking and getting to know the contexts that "trigger" the use of subjunctive, rather than trying to predict them from a general concept of what subjunctive forms tend to mean.
Thank you very much, Francisco! Your explanations made a lot of sense. I think your approach of learning the various contexts that trigger the subjunctive is the best way to approach learning the mood.