asks: Any tips about subplots? I feel I'm lacking in them.

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asks: In books there are obviously side characters and smaller problems (lost keys, going to the store for groceries), etc. that normally don't really affect the ending. How would you balance between main storyline and those smaller problems and side characters?

The primary plot of the story involves the main character and their main goal. Everything else is a subplot. In The Selection, the main plot is America Singer's involvement in the princess selection process of the royal court. One subplot is dealing with her feelings for her boyfriend. Her friend's illicit romance with a guard is another. The evolution of her friendships within the selected are other subplots.

The purpose of a subplot is not to add filler to a thin story. It's to enrich it, give it color. A subplot may or may not affect the ending of your story, but it can make the journey more interesting. What a subplot should not do is overtake the story itself by becoming the primary focus. How do we find that balance?

A subplot is when there's a goal other than the main plot goal. It could be the best friend's struggle with sobriety. It could be the mother's attempt to reenter the workforce. It could even be the dog's efforts to escape the house. How the main character interacts with these side characters gives the reader additional insights to their personality, and these interactions can involve the side character's personal goals.

Some side characters might have big personalities and attempt to take over your story. Don't let them. Always remember who the main character is, and what their primary goal is. When the story moves forward, it should always be in that direction. If a subplot doesn't involve the main character at all, then ask yourself what its purpose is. If it's to enhance the main plot somehow, then good. If it's to give a side character a bigger role, then you might want to rethink why they need a bigger role. If more than one chapter at a time is being dedicated to a subplot (in other words, you have multiple chapters in a row), then it might be too much. Try to space them out so we don't lose sight of the main plot.

I'm honestly not an expert on subplots. My own stories tend to have very few of them. But I hope I've said enough here to answer your questions.