Subplots

A main plot is who wants what, and what refuses them. A subplot is another want, belonging to the same people or to someone close enough to matter, that keeps crossing the main road. Love beside a crime. A family debt beside an ambition. A friendship that cannot survive the choice the hero is about to make.

The subplot earns its place by changing the main story. It supplies a clue, a temptation, a cost, or a mirror. If you can lift it out and the ending is identical, it was an anecdote. Either braid it in until the ending needs it, or save it for another book.

Practically, the subplot is how you delay an answer without stalling. End a chapter on the brink, and spend the next chapter on the other thread. The reader is impatient in a useful way. They will tolerate your second story because they want to get back to the first, and if you have built the second story properly they will be glad they were made to wait.

Do not start a third and a fourth thread because you are bored. Two, sometimes three, are enough for most novels. More than that and the reader starts a notebook. You have written a chart, not a book.