Some HDRIs have a sun, does this replace the directional light? Or do you keep both?
No it doesn't replace the directional light. You can keep both, but if they're on opposite sides of each other it may look weird to look up and see a sun like object in a darker area of the HDRI skybox. If you do keep both I would recommend trying to keep the image based sun in the same area as your directional light.
watch the tutorial on environments I think it is in the lighting class, it is not explicitly labeled if I remember correctly though