Plot is past the moment. Subplot is the defining moment of a character's life that explains the reaction properties, the strong virtue or flaw in their make-up mixed with action, emotion or circumstance. Many people define Calamity as plot but the truth of the matter is that it is part of the journey. It is the cobblestones that you write to define the path of your characters till the audience sees all the blocks turned to smooth concrete in their imagination. I can tell you that there is one blatant difference between the two.
Calamity has a choice to be a moment without closure.
Plot must be resolved. The Hollywood formula has changed over the years to add an addendum to end the plot with the indications of a sequel. So your ending needs to be the indication of a new story or climax. Before you relive the anxiety of grammar school travesty starring at the blinking black line on your writing program or playing the shooter game your cousin left on the system two years ago in frustration it is very simple. If you know how to differentiate the two all you have to do is maintain one subplot element that is not resolved.