I recently replaced the ceiling fan is my very old house. The original fan worked properly, had separate fan and light switches on the fixture, and was controlled by the light switch on the wall which also operated properly. When I dismantled the fan, I found two main electric cables coming from the ceiling instead of only one. I assumed the two black wires were connected to the fan's black wire and the two white wires were connected to the fan's white wire so quickly disconnected it all without noticing closely.

When I installed the new fan, I put the two black wires from the ceiling with the black and blue fan wires and put the two white wires with the fan's white wire. When I turned the power back on, the fan now turned on when the light switch (on the wall) was in the off position. The fan operated properly until I turned it off and this flipped the breaker.

Can anyone please advise on what I do with that extra main cable coming from the ceiling?

GM

Yep, that's what happens when you don't codes one of the white wires with black tape. One of the cables is hot all the time and the other is known as a switch loop. Its white should be taped black. Connect only one cable to the fixture until it works with the strings then disconnect the black and connect to the white of the other cable and tape the end black. Take the black of the other cable and connect to the black/blue of the fixture. Replace wall switch because you may have let out the magic smoke!

One of the black wires is hot. The other goes to the switch. You problem is to determine which is which. Buy yourself a voltage detector. Separate all the wires and put wire nuts on them. Turn the power on. Hold the voltage detector next to each black wire. It will indicate which has current.
Turn power off. Connect the hot black, the fan black and the white wire of the other cable going to the switch together with a wire nut. Connect the remaining white wire and the fan white wire. Connect the remaining black wire and the fan blue wire.

Did you get this working?