/gamemode

Sets a player's game mode to survival, creative, adventure, or spectator.

Syntax

/gamemode <gamemode> [<target>]

Name	Type	Required	Description
gamemode	gamemode	Required	The game mode to set: survival, creative, adventure, or spectator.
target	entity	Optional	The player(s) to change. Defaults to self.

/gamemode creative

Sets your own game mode to creative.

/gamemode survival @a

Sets all players to survival mode.

/gamemode spectator @p

Puts the nearest player into spectator mode.

Using numeric mode IDs (0, 1, 2, 3), Java Edition 1.13+ requires the full name (survival, creative, adventure, spectator).
Forgetting that switching from creative to survival removes the ability to fly, causing fall damage if airborne.

Switching to creative while falling negates fall damage. Switching from creative to survival while flying will cause the player to fall.

Try these commands on Astroworld or host your own server.