How do I stop a toilet from constantly running?

Lift the toilet tank lid. The most common cause is the flapper (the rubber seal at the bottom of the tank) not sealing properly or the chain connecting the flush lever to the flapper being too long (getting caught under the flapper) or too short (pulling the flapper up slightly). Check if the flapper is aligned correctly over the drain hole and if it's worn or warped. Adjust the chain length so there's only slight slack when the flapper is closed. If the flapper looks damaged, replace it.

If the water level in the tank is too high, it constantly overflows into the overflow tube, causing the fill valve to keep running. Locate the float (either a ball float on an arm or a vertical float cup). Adjust the float mechanism (usually a screw or clip) so that the fill valve shuts off when the water level is about 1/2 to 1 inch below the top of the overflow tube.

If adjusting the float doesn't work or if the fill valve itself is leaking or malfunctioning (e.g., not shutting off water completely), the entire fill valve assembly may need replacement. Turn off the water supply valve behind the toilet. Flush the toilet to empty the tank. Use a sponge to remove remaining water. Disconnect the water supply line from the bottom of the tank. Unscrew the locknut holding the fill valve in place and remove the old valve. Install the new fill valve according to manufacturer instructions, reconnect the supply line, and turn the water back on, checking for leaks and proper water level.