The subjunctive mood is not very common in English. It was, but now it has lost most of its importance.
However, it is still used in the following cases.
a) When the dependent clause expresses a wish.
I wish I were prettier.
I wish she were here.
b) In if-clauses that express an unreal or imaginary condition.
If he were more diplomatic, he would make a better leader.
c) When clauses beginning with as if and as though describe a speculation contrary to the fact
She behaved as if she were mad.
d) In that-clauses that express request, demand, requirement etc.
I requested that she be present at the meeting.
The present tense of the subjunctive uses the base form of the verb. That means when the subject is a singular noun, the verb does not take the marker –s.
She demanded that her son accompany her to the estate.
(NOT She demanded that her son accompanies her…)
I insist that I be given my dues.
The past tense of the subjunctive uses the verb were regardless of the number of the subject.
If she were taller, I would marry her.
He wishes he were richer.
Of course, was is possible here. However, in that case, the sentence will not qualify to be in the subjunctive mood.
If she was taller, I would marry her.
If you listened to my advice, you wouldn’t be in this trouble.