A recent and popular self-help book wryly notes that if adolescence

Here is another explanation:

subjunctive mood (indicating a hypothetical state or a state contrary to reality, such as a wish, a desire, or an imaginary situation). It is harder to explain the subjunctive. Five hundred years ago, English had a highly developed subjunctive mood. However, after the fourteenth century, speakers of English used the subjunctive less frequently. Today, the mood has practically vanished; modern speakers tend to use the conditional forms of "could" and "would" to indicate statements contrary to reality. The subjunctive only survives in a few, fossilized examples, so they can be confusing. Here are the most common uses:

1. By far the most common use of the subjunctive is the use of the subjunctive after "if" clauses that state or describe a hypothetical situation.

Subjunctive: "If I were a butterfly, I would have wings."

Note that in the indicative, we normally write, "I was." For instance, "When I was a young boy, I liked to swim." However, to indicate the subjunctive, we write "I were." The subjunctive indicates a statement contrary to fact. In the butterfly example above, I am not really a butterfly, but I am describing a hypothetical situation that might occur if I were one.

Indicative: "When I was a butterfly in a former life, I had wings."