The subjunctive mood is one of the more difficult tenses in the English language. The subjunctive mood uses the past tense or the past perfect tense to indicate a wishful thinking or an action that is not likely to happen or did not happen in the past.
e.g. If I were you, I would accept the offer. (i.e. the offer is good, do accept it NOW)
e.g. If I were you, I would have accepted the offer last week. (i.e. you didn't accept the offer)
e.g. If you had called the doctor, the patient would have lived. (i.e. you did not call the doctor; the patient did not live)
e.g. If pigs had wings, they would fly. (i.e. pigs don't have wings, and that's why they don't fly)
e.g. If he has the money, he will help you. (i.e. he may have the money; if he does, he will certainly help you)
e.g. If he had the money, he would help you. (i.e. he doesn't have the money NOW; therefore, he will not help you)
e.g. If he had the money, he would have helped you. (i.e. he didn't have the money, and that's why he didn't help you)
Remember this: using past tense for a present action indicates the improbability of that action, while using the past perfect tense, the improbability of that action in the past.
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