I have an arduino Nano
i used a 5V regulator from a 9v battery to supply the power to VIN instead of +5V and i used the +5v pin to power something else
5V into Vin won't hurt anything. It's normal to get some voltage drop across a regulator. In fact you need some voltage drop across the regulator in order for it to work... So, you need to put more than 5V into a 5V voltage regulator.
There is a spec is called "dropout voltage". For example, if the dropout voltage for a regulator is 1 Volt you need to feed at least 6V into a 5V regulator or else it "drops-out" of regulation and no longer functions as a voltage regulator.
The wording is awkward and not real clear, so it's hard to give advice. A drawing is what is needed even if it's handwritten, picture taken and posted. Just to be clear. applying +5vdc to the Vin pin is not a proper voltage, it's too low. Applying a regulated +5vdc to the 5V pin is OK.
If you put 5V into Vin (the same as the barrel jack on an UNO, etc) then you will get less than 5V out of the 5V pin, and the voltage regulator won't be doing its job.
You can put regulated 5V into the 5V pin and nothing in the Vin and it will be fine.