A subplot is “a subordinate plot in fiction or drama” according to Merriam-Webster. But what does that mean? Well, it’s a story told alongside the main story in a novel or drama. Think of it as the story of what characters are doing that have nothing to do with the events of the main story. In real life, we don’t focus on one event in our lives. We need to plan a party, but we still have to go to to work or do our normal daily chores. The party planning is the main story line, but our regular life is the subplot to that story. The two stories are independent of one another, yet connected.
Subplots can beef up an otherwise thin story. That’s not their only advantage. They can increase tension the main story by taking the reader away from it at a critical time. This serves two functions. It lets the reader take a breath, while making her wonder what will happen with the main story. It breaks tension and increases suspense. They also add depth to the story. They create the illusion that the characters have lives outside the main story.
You can have subplots that involve the main characters. You can use one to let minor characters shine and let your main characters play minor roles for a bit. It’s your choice how to handle them. You can weave the subplot into the main story or let it run parallel to the story. Again, it’s your choice how to handle your subplot.
Let’s go back to the party-planning example. You have characters planning a party of some sort. Now, if you were planning a party, you wouldn’t do it all the time. You would still need to go to work, sleep, eat, and so on. This is where the subplots can help you break up your main story – party planning – in a realistic way. So your character, let’s call her Ann, is planning a surprise party for her boyfriend, Josh. That’s a story goal. She needs to hide her activity from Josh. She can’t take time off work to plan the party, she works with Josh who would wonder what she was doing. So she must sandwich the party planning in on her lunch hours, breaks and before and after work. That’s how you put in the subplots.
Subplots can also be a good source of back story. You can put a subplot in as a block of back story. You can use a short story within a novel to explain the reason for your story’s main plot. Be careful in using this version. You don’t want an information dump.
How many subplots should you use? I would say keep it simple. Don’t add more than you can handle. I usually add one or two because two is all I can handle at one time. You also don’t want to irritate your readers by jumping around like a water drop on a hot griddle.
So all you need to do is tie up all loose ends. Be sure to conclude the stories to your (and presumably your readers’) satisfaction. It’s best to tie up the subplots before the main story. That’s only for your own convenience.