when it is necessary to use the subjunctive mood.
The Subjunctive is not a tense but a mood. What does that mean? It is called a mood because it doesn't deal with factual reality but with opinions, feelings, suppositions, dreams and speculation. We use the Subjunctive to mentally and emotionally organize our world in terms of others.
Subjunctive mood. The subjunctive is a grammatical mood (that is, a way of speaking that allows people to express their attitude toward what they are saying) found in many languages.