I am confused about the subjunctive tense

The subjunctive is NOT a tense but a mood.

There are 3 moods that you need to be aware of (and there are tenses within each mood):

The indicative mood, which is all about reality: real actions, events, thoughts, beliefs, facts and objective statements. 

You need to know the present, preterite, imperfect, perfect, future and conditional tenses of the indicative mood

The subjunctive which is all about unreality: subjective opinions, hopes, feelings, doubts, what you want or advise other people to do, and hypothetical statements.

You need to be familiar with the 4 types of subjunctive (present, imperfect, pluplerfect, and perfect).

The imperative mood which is about giving orders

You need to know how to form the imperative and also the negative imperative (which uses the subjunctive form - see table from question 1)

I have a spreadsheet that has more information on verbs, mood and tenses, which can be used to walk the student through this knowledge.