how do you stop the player from dying

Erm, that's actually pretty vague. It depends what the player is dying from and under what circumstances you want it to stop. Maybe you could use a procedure to add health or apply a potion effect?

You can use the command: /effect @p minecraft:resistance [time] 255 or you can use the procedure: add potion with level 255 and duration [time] to provided entity: resistance (under entity procedures)