Subjunctive mood usually starts with if. It seems: If you are happy/if you were happy (both are right) If you were me (right) / if you are me (wrong) If you want to learn English / if you wanted to learn English (which is right?) Which is subjunctive mood?
This isn't a very clear-cut topic, so the best thing to do would be to read the materials for the particular course you're following (if any). Also, why would you need to know?
The form to which the term "subjunctive" is most commonly and least controversially applied is the use of the plain form of the verb (identical to the bare infinitive) in mandative constructions such as:
- She ordered that he stop.
- He demanded that it continue.
- We asked that she leave.
These mandative forms are commoner in American English than British; British English prefers either to re-word ("She ordered him to stop") or to insert "should" ("He demanded that it should continue") - although the use of the mandative subjunctive may be increasing under American influence.
Subjunctives of this kind are also found in a few set-phrases ("So be it", "Be that as it may", "Long live the Queen"). Expressions such as "Whether it be red or blue" are also occasionally used (though "is" is at least as acceptable and more common).
The "irrealis" is the use of "were" instead of "was" in counterfactual statements related to the 1st and 3rd persons singular:
- If I were you, I wouldn't worry.
- If she were here, you would be happier.
- If that were the case, I would tell you.
In traditional grammar, the first type of subjunctive is called the "present subjunctive" (because it is the same in form as the present tense, except in the 3rd person singular) and the second type is called the "past subjunctive". Nowadays, these terms tend to be avoided because they are seen as confusing. The "past" subjunctive has nothing to do with past time (although neither does the use of the past tense in sentences like "If he visited, she would be happy"), and the "present" subjunctive isn't always associated with the present, either.
Traditionally, some would regard the forms in "She asked that I stop", "If you were here", "If she kissed him" as subjunctive too. In this perspective, there are a large number of forms that are ambiguous between indicative and subjunctive - this is called syncretism.
In modern grammar, the term "subjunctive" tends to be reserved for forms that are visibly and distinctively subjunctive, not syncretic ones that are assumed to be subjunctive. (Additionally, in modern English, it is possible to replace irrealis "were" with "was", especially in colloquial use, so it would be odd to assume that a non-distinctive form were subjunctive.)
1: If you want to learn English, ELL is the site to use.
2: If you wanted to learn English, ELL would be the site to use.
I couldn't really argue with anyone who wanted to swap is / would be in those examples. The versions as given are certainly more likely, but it's not obvious to me that the alternatives would be "ungrammatical".
But consider a context like ringing through to room service in a hotel, where the person who answers might say...
[Hotel reception speaking...]
3: ... what do you want?
4: ... what did you want?
Both Present and "Past" are perfectly idiomatic there, but I've used "scare quotes" because I see no good reason to classify my example #4 as being significantly different to #2.
Maybe some people would say #2 is "Subjunctive", whereas #4 is "Past", but that doesn't seem a useful distinction to me.
I think English only really has 2 tenses (Present and "Not-Present"), and #4 is a typical example of "Not-Present" being used to "distance" the speaker from the utterance (introducing "hesitance, deference", characteristic of many formal contexts). A bit like asking Who might you be? rather than Who are you?, for example.
TL:DR: Using "Not-Present" verb forms in English doesn't necessarily have anything to do with time-based relationships. Example #2 above is just a common way of making the assertion more "politely tentative" (arguably implying more strongly than #1 that perhaps you might not want to learn English anyway).