A single wall switch controls the power to the fan and light together whereas a dual wall switch has two separate switches one for the fan and the other for the light.
Exhaust fan and light on separate switches.
Assuming you have 3 wires from the switch box to the fan light location as bill said.
Take the 2 wires on the switch that controls the outlet off the switch twist.
The power source will either.
Attach the red and black wires from the fan light unit to the top terminal screws on the switches.
Connect the two white wires in the switch box together with a wire nut.
How to wire your bathroom so the lights and fan are on separate switches the electrical wiring for separate switching of the bathroom exhaust fan and the light fixture will require identifying.
Then this is what you do.
Light timer switch ul listed fosmon 9 setting 1875w in wall countdown electrical switch off 5 10 15 20 30 min 1 2 4 hr energy saving for fans vent lights heaters decorative 2 pack 4 4 out of 5 stars 166.
If the existing wiring has only 1 switch for control and the power comes to the switch first then you can only control power to the ceiling fan assembly via the switch and you have to control the fan and the light with the pull chain switches when the wall switch is on.