You should pour bleach down the pipe.
the hot water valve shut off valve shuts off the hot water. The cold water valve shuts off the cold water. ;()
A water Toby is a shut valve at a water main
The valve seat or washer on the main valve are not seating/sealing properly.
On any water heater, the shut off is on the incoming cold pipe.
Outside at the main shut off valve. Showers/Bathtubs do not normally have a shutoff valve like sinks and toilets. The main water valve for the house must be turned off.
A non-positive shut off valve is one that is controlled by the user. You hold down the valve to keep the water or whatever coming out, and when you want it to stop, you release the valve.
If the shut off valve turns but does not shut the water off, the washer in it has split and probably made it to the faucet. It was blocking part of the hole on the hot side and at some point either shifted or made it through the faucet and in in the screen in the end of the spout. Shut the main water off and replace the washer in the shut off valve.
An air-intake engine shut-off valve regulates air-flow to the motor, and operates as an emergency shut-off valve. There are also shut-off valves on water hoses, so shut-off valves really control the inflow of any liquid or gas.
With the water main shut of valve of course
The inlet valve is shut off on the cold water feed side or your mixing valve or tempering (if you have one has failed.
I do not live in a condo thus I have no need to shut the main valve
It is possible but not very common. Usually a bad carb will cause a non-start issue or it will shut off within minute or two if it has a stuck float valve causing it not to get gas.