you give a player effect by doing /effect <effect>

You can use /gamerule commandBlockOutput false to stop commands appearing on screen.

use /gamerule spawnRadius 1 to keep the world spawn actually spawn the player on a radius of 1 block

/give <Player> <Item> [​Amount] Used to give a player an item.

/gamerule keepInventory true Keeps your Inventory when you die.

/atlantis Raises the world’s water level

/ride Turns the creature you’re facing into a mount.

/dropstore Stores all inventory items into a chest, which spawns nearby.

/freeze mobs stop moving

/instantplant makes plants grow in a instant.