The subjunctive mood is used primarily to express a wish, such as, "I wish you were more careful with the vase." However, it is also suitable to use when describing an untrue situation, such as, "If John were more talkative, he'd have more friends," a speculation, such as, "He speaks as if he were the true killer," or even a request, such as, "I requested that you be present at the dinner table."
The subjunctive mood has two tenses: present and past. You can form the present subjunctive by dropping the "-s" from the third person singular. For example, "walks" becomes "walk" and "fights" turns to "fight." "Is" and "are" become "be." The past subjunctive is the same with the past indicative. For instance, "walked" remains the same, but the verb "be" becomes "were" for the first, second and third person -- singular and plural.
When attached to independent clauses, certain verbs and adjectives indicate that the clause has a verb in subjunctive mood. These verbs include "command," "insist," "demand," "order," "require," "recommend," "suggest" or "wish." For example, "He recommended that I went home." The adjectives serving this purpose generally express urgency and include "essential," "crucial," "imperative," "important," "necessary" and "urgent." For example, "It is imperative I go to the bathroom."
As subjunctive mood is not commonly used in everyday speech, many speakers tend to disregard it. Therefore, it is quite common to hear "If I was you, I would..." For someone who has never heard of the subjunctive mood, this sentence abides perfectly by English grammar rules. Another usual mistake is when after the aforementioned verbs or adjectives, people use indicative mood: "I suggest that he moves the car" instead of "I suggest that he move the car."