The subjunctive mood is slowly disappearing; you seldom need to use it. But seldom is not never; there are situations in which you must use the subjunctive. So, you need to know what those situations are. Stay with me; it’s not difficult.
Overview of the major moods
“There are three major moods in English: (1) the indicative mood is used to make factual statements or pose questions, (2) the imperative mood to express a request or command, and (3) the (rarely used) subjunctive mood to show a wish, doubt, or anything else contrary to fact.” (Source)
Examples (all are song titles from famous musical comedies)
Indicative: “A Secretary Is Not a Toy."
Imperative: “Get Me to the Church on Time.”
Subjunctive: “If I Were a Rich Man."The Takeaway: If you use the indicative where you should have used the subjunctive, your readers or listeners will normally guess what you meant to say. However, the well-educated people among your readers or listeners will think you are ill-educated. Therefore, to avoid losing credibility, you must learn when to use the subjunctive. If, following my advice, you have been reading good writing aloud for at least ten minutes a day, eventually you will learn the subjunctive automatically, by unconscious imitation. If you want faster results, study the examples here and here.