The sub-plot is dependent on the main plot, although it may not seem so at the time – it might provide twists, shows us how the main character reacts in other settings, provides depth to the story.

They often involve minor characters.

They move the story along toward an apt ending and help the reader get to know the characters better.

Very few novels can survive on one plot alone. Only two come to mind:

My Name is Leon by Kit de Waal - a boy is put into foster care but misses his baby brother desperately

Chesil Beach by Ian McEwan - a honeymoon night ruins a marriage

Most novels have at least one main plot and one sub-plot:

The Children’s Act, Ian McEwan – the court case is the main story, although the judge’s marriage and her husband’s infidelity provides us with distraction.

The Vanishing Act of Esme Lennox – the main plot takes place in Esme’s past, while the subplot takes place in a different time.

Esme Lennox – the sub-plot is via time. At the beginning of the novel Iris seems loosely connected to Esme but we follow both women’s stories to find that they are related.

The Goldfinch by Donna Tartt – Theo is looking for someone to love him like his mother did is main plot, but other characters let him down provides other stories that intrigue us.

The Vanishing Half by Brit Bennet tells of two sisters and the colour barrier in the American south. We follow one sister as she ‘passes’ for white; the other family provide a counter-balance, the difficulties, to that life.

The Dutch House by Ann Patchett, a tale of a house and a family. That the family fall apart because of the house is undoubtably the main plot, but the house provides a solid sub-plot to all that occurs.

Circe by Madeline Miller, puts flesh on the lives of Greek gods and immortality. Could we be content to never work and only play? Would life really be better if we never aged? While Circe’s banishment to a lonely island is the main plot, her human interactions with Odysseus is a strong sub-plot.

Use the sub-plot to heighten tension by bringing in other characters with their own conflict and dilemmas.

They can provide surprise:

The Goldfinch – bombing, father turning up, betrayal, finding Boris again as an adult, the painting …(no spoiler)

Each conflict reveals more about the plot/characters/background. This can provide twists that create tension, making your reader think, 'How did this happen?' or 'What now?' 

A sub-plot should not feel contrived or superimposed on your story.

The pace can be defined by how many twists/sub-plots are in the story – more increases the pace, fewer slow it down.