→Traditional English grammar:
  • Infinitive events (like: do): At the start of event it's triggered.
  • Past participle events (like: done): On the completion of event it's triggered. 
→Types of Events
  • When click on mouse or drag.
  • When press any keyboard key.
  • Automatically run events after specific program/function execution.
→Event
  • Suppose when you click on a mouse then an event occur. It doesn't mean only one function or procedure execute for events. May be group of programs run for specific events.
preventDefault() functionality:
  • All infinitive events provide preventDefault() functionality. 
  • Can stop the execution of an action before it starts. 
  • Return false when called [return e.preventDefault()].
Ref:https://getbootstrap.com/docs/4.1/getting-started/javascript/