Most writers have a basic understanding of plot: It’s what the protagonist goes through while pursuing her goal. Subplot seems to be less obvious. Recently I’ve read a few unpublished and self-published manuscripts in which the writer did not seem to understand the function of subplot. In one case, a subplot was omitted entirely, making for a very cut-and-dry book.
The most basic description of subplot is that it encompasses the subtle obstacles that keep the hero from realizing his goal. In other words, the subplot is not a completely different set of circumstances from the main plot. For example, if the main plot was about a woman who was working toward being an astronaut, the subplot might be about her relationship with the boyfriend who didn’t want to leave their small town. Or caring for her once-brilliant father, who now has Alzheimer’s. Both such subplots subtly raise the possibility that these relationships will keep her from pursuing or ultimately realizing her goal. On the other hand, a subplot about her sister nursing a wounded deer would only detract from it. (OK, there are probably some strong writers who could make the astronaut/wounded deer thing work. But why make things harder than they have to be?)
Just like the main plot, the subplot must have a concrete beginning, middle and end. It has to have conflict. It cannot be a series of scenes that, say, take place at the heroine’s place of employment but never really go anywhere. That’s not a plot; that’s a diversion.
Some Rules About Subplotting
Have plot points that impact the main plot;
Show the protagonist in a different role than in the main plot;
Be centered around relationships (if the story is not a romance), or be centered around something other than relationships (if the story is a romance);
Resolve before the main plot does;
Interact naturally with the main plot.
Subplots Should Not:
Be completely separate from the main plot;
Show characters acting in very different ways than in the main plot (unless that is the plot);
End less than halfway through the book;
Be composed of coincidences and deus ex machina (this is true for the main plot);
Be comprised of a completely different set of characters than the main plot.
Converge neatly with the main plot and resolve at the same time;
Begin before the main plot;
Show the protagonist reacting, rather than acting.
Writers who are “pantsers” rather than plotters may find their subplots get away from them more than the main plot does. If so, a strong rewrite should be done to tie the two together.
Like all guidelines, these work about 80 percent of the time. Then 20 percent of the time, some hot shot writer will come along, break all the rules, and do an amazing job. (You are not that hot shot writer. Neither am I.)
The best subplots are so well interwoven with the main plot that it takes careful teasing to separate them. This is often the case in genres such as literary fiction. In more mainstream work, such as mysteries, the plot and subplot are more easily distinguished (the plot having to do with the mystery the detective is trying to solve, and the subplot having to do with an entanglement in her personal life that keeps getting in the way.)
While most writers rightly focus on their main plot, books are made or broken on the strength of the subplot. If you ignore it, it will come back to haunt you.