By convention, a story is not a novel unless it has at least one subplot.  Without a subplot, a story is generally considered a novella, a short novel with a single narrative if you will.  Subplots are secondary stories to the main plot line.  To be successful, they must influence or contribute to the main story in some way.  Subplots can chronicle a romantic relationship, often involving the protagonist, describe the course of another relationship, contribute to a theme of the main story, or develop a character.  Subplots may assume more than one of these roles, for example, both revealing the impact of a relationship on the story and enhancing the theme.  They deepen the novel.  Subplots may also cause the protagonist or other characters to make decisions that influence the main plot. 

If a subplot does not contribute to a story, it needs to be eliminated.  That is a solid general guideline:  If something does not contribute to the main story of novel in some way, it needs to be eliminated, changed or replaced.  As a professor once told me, what you omit can be as important as what you include.  Subplots are one of the scores of times with a novel where you must exercise judgment. 

Although it’s on a smaller scale, a subplot needs several of the aspects a plot does.  As with a plot, a subplot at least needs a character with an objective, an obstacle or opposition, and a struggle and climax leading to a resolution.  Like a plot, a subplot’s story must engage the reader.  Because a subplot is not as involved as the main plot, it may not involve a personal flaw or an arc of character change, although those may be useful.  It may not have a triggering event because the objective may be long-standing.  A subplot will not involve as much complication. 

Still, as with the main story, a subplot may need some of the same features to keep it moving.  It may need turning points, and it may need a midpoint in terms of an emotional realization.  

Note that a relationship can have importance even if it is not a subplot.  On the other hand, a relationship can certainly be the basis for a subplot.  Whether a relationship is a subplot or not depends on whether it meets the minimal standards for a plot.

In terms of the number of subplots, there likely should be one, two or three.  A single subplot may make the novel feel skimpy.  More than three subplots is much to handle, as the subplots must connect to the main story and sometimes with each other.  Too many subplots can detract from the impact of the main story.

If you don’t have them in mind already, you create subplots with the same techniques used with the main plot.  You watch for wisps of ideas.  Here you have a start already, since thinking about your story may prompt wisps.  Ask questions. 

Would the protagonist having a romance, a friendship, a rivalry, or a boss of some sort add interest to the story?  What sort of relationship is it?  What is the protagonist’s goal with this relationship, what are the obstacles, what is the struggle and the climax? 

Would a substory to illustrate part of the protagonist’s character be interesting?  What needs to happen and who needs to be involved to show and explain that part of the protagonist? 

In terms of a thematic subplot, what comments on a theme would you like to make that the main story may not show?  This could be an expansion of part of a theme, a qualification to it or even a counterargument to that theme. 

As always, the question is what will entice the reader, draw the reader further into the story. 

Develop those wisps into full ideas by letting yourself brainstorm whatever comes to mind with your internal editor firmly locked up – often doing this with paper and pen will help — then later considering, altering and possible mixing the best wisps.  Also use directed fantasies:  Picture some portion of the subplot – it can be anywhere in the chronology – and imagine what might have occurred – or needed to occur — earlier and what might happen later.  Consider how the subplot idea will interact with or amplify the main plot.  Continue working back and forth until you have solid subplot ideas, even if you don’t know all the specifics of them yet.

Read more: 6.  Selecting Subplots