A one switch ceiling fan with 2 white and 2 black wires from the ceiling. How do we wire fan? The switch has only one black and one white wire. The fan has one white, one black, and one black/white wire.

A one switch ceiling fan
What does that mean? Because you have three wires coming from the fan I am inclined to believe that this is a combination fan and light. If it is a conbination fan/light and has only one switch, it requires a remote control module. Wiring for a remote controlled fan/light will be different.

If the fan/light has a pull chain to turn on the fan and a pull chain to turn on the light you can used the following to connect it.

If the switch box has one black wire and one white wire ONLY, and the ceiling box has two black wires and two white wires ONLY, then one cable (one black wire and one white wire in the same outer covering) is bring power to the ceiling. The other cable (one black and one white) is connected to the switch.
You must determine which cable is the power cable and which is the switch cable. If you have an electrical tester of some type, you can do that by checking for voltage between the black and white of each cable.

If you do not have a tester turn the power off at the breaker. Temporarily connect both the black wire from the fan and the black/white wire from the fan to one of the black wires from the ceiling. Connect the white wire from the fan to the white wire in the same cable.

Turn the power back on at the breaker. If the fan/ light works, (don't forget to pull the pull chains) you have connected it to the power cable. If nothing works you have connected it to the switch cable.
Now that you know which cable is the power cable and which cable is the switch cable, turn the power off. Put a piece of black electrician's tape on the white wire going to the switch to mark it as the switch cable.

Connect the black from the fan, the black of the power cable and the white (with the black tape) of the switch cable together with a wire nut. Connect the black/white from the fan to the black of the switch cable. Connect the white from the fan to the white of the power cable.

The switch on the wall will control the light, again don't forget to turn the light on with the pul chain.

The fan pull chain will control the fan.

Which black wire will carry the voltage? Which black wire will i connect to the light after testing it with the tester?

WHICH BLACK WIRE WILL CARRY THE VOLTAGE? WHICH BLACK WIRE WILL I CONNECT TO THE LIGHT AFTER TESTING IT WITH THE TESTER? I have te black and white wire for the light connected to the 2 black now and I must use the chain despite have 2 separate switches

NLB416, what are you talking about? We have no idea what you have anywhere. You are posting this as if we were standing there looking at it.

How was the old light connected? Do the same thing.