Does a trigger that says 'whenever you draw a card' fire when a replacement effect replaces the draw?

A triggered ability that says "whenever you draw a card" watches for an event: a card being drawn. Under CR 603.2, a triggered ability triggers only when its trigger event actually occurs. If a replacement effect (CR 614.1) intercepts and replaces the draw with a different event entirely, the draw never happens, and no trigger fires.

The key distinction is between replacement effects and triggered abilities. Replacement effects (using "instead") modify or substitute the event before it occurs, meaning the original event — the draw — is never completed. Since the draw event never happens, there is nothing for a "whenever you draw a card" trigger to see (CR 614.5).

However, if an effect triggers off a draw and then does something different (e.g., a triggered ability that says "whenever you would draw a card, instead..."), that is not a replacement effect and the timing works differently. Only true replacement effects using "instead" prevent the draw from occurring.

Concrete example: You control Teferi's Puzzle Box and Reckless Scholar. Puzzle Box says "whenever a player draws a card, that player puts it on the bottom..." — this is a triggered ability, not a replacement effect, so the draw still happens. But if an effect said "if you would draw a card, exile the top card instead," no draw occurs and Puzzle Box's trigger would not fire for that event.

In summary: a genuine replacement effect that substitutes something "instead" of a draw prevents the draw event from existing, meaning CR 603.2 triggered abilities watching for draws will not trigger (CR 614.1, 614.5).