Some queries

What are the things that beginners do and by this their boards stop working

Powering their boards directly with 120 AC.

By publishing detailed description - what was done and what are the current symptoms.

Drawing more current from the IO pins than they're rated for (including short circuits).

Subjecting IO pins to higher voltages than they're rated for.

Connecting inductive loads like motors, solenoids, relays directly to IO pins, thus subjecting them to flyback voltage spikes.

Breaking the USB micro jack off of the boards.

Here's some relevant information "10 Ways to Destroy An Arduino":

Can I connect 9 v DC power to it

What is "it"?

Can I connect 9 v DC power to it

Maybe. But often a better question is should you.

I am just asking that should I connect 9 V dc power to uno

You can, on a couple of conditions.

You can connect it via the "barrel jack". As long as it is 9 V, not 12 V or more as it might be with an older 9 V "plug pack" or "wall wart" which has just a transformer, rectifier and capacitor.

It needs to be the right polarity - centre spigot is positive, outer sleeve negative. If it is the wrong polarity, it will not work but the reverse polarity protection diode will protect the electronics.

You can connect it with positive to the "Vin" pin and negative to "GND". In this case, if you connect it wrong and put the negative to the "Vin" pin, you will burn out the Arduino.

And the on-board regulator which drops the "Vin" to a regulated 5 V can only supply the Arduino itself and a couple of indicator LEDs at 20 mA or less, each.

If you want to power other devices and modules which require 5 V, you must provide a regulated 5 V power supply which you then connect with the correct polarity, to the "5V" pin and ground as well as to those other devices. You do this instead of using the "Vin" pin. If the total requirement is 500 mA or less, you can power the Arduino through the USB port with a 5 V USB "phone charger" which would be fairly easy to come by.

Thanks sir