Hard vs soft light
How sharp the shadow edges are. A small light source gives crisp hard shadows; a big close one gives gradual soft ones.
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What it is
The only thing that decides hard or soft is how big the light looks from where the subject is standing. Hard light comes from a small apparent source and draws shadows with a knife edge: bare sun at noon, a bare bulb, a phone flash. Soft light comes from a large apparent source and lets the shadow fade out gradually: an overcast sky, a big window, a softbox pushed in close. The sun is enormous and still hard, because at that distance it reads as a tiny dot.
Pick hard when you want texture, edge, and graphic shadow shapes: fashion, sneakers, anything where the shadow is part of the design. Pick soft when the subject should not fight the light: skin, food, glossy packaging, most portraits. Same modifier, different result, just by moving it: a softbox at two feet is soft, the same box at twelve feet is basically hard.
Gotcha: slapping diffusion on a light does not make it soft on its own, size and distance do most of the work. And soft is not automatically flattering. Wrap the light evenly from every side and you erase the shape of the face and it looks flat and cheap. What reads as good lighting is usually soft light with a clear direction and shadows that are still allowed to exist.
Ask AI for it
Light this subject with a single large soft key: a 4-foot softbox placed close, about 45 degrees off-axis and slightly above eye level, with a white bounce card on the opposite side to lift the shadows about two stops. Shadow edges should be gradual and wrap around the form, with no hard shadow lines and no visible second shadow. For the hard-light version instead: one small bare source, direct and undiffused, crisp shadow edges with sharp outlines, deep contrast, and no fill.