i am beginner driver, sometimes when i shift into second gear too fast ,i didn't push stick it all the way and after release the clutch you could hear the scratch sound ,stick vibration and after few seconds gear popped out into neutral.i don't do all the time, but on occasion it does happen at my fault. could this be cause to kill synchro ring ?
If you can get the gear to "stick" on the synchro ring and try to drive the car until it pops out of gear, I think either you are holding the gear stick in position (which is a bad habit) or the gearbox must be getting worn or badly adjusted.
If the synchro is working properly, it should "pull" the stick fully into gear, unless you are gripping the stick tightly. Learn to just "push" the stick in the direction you want to move it, and don't grip it tightly with your fingers or keep your hand on it after you have moved it.
If you are changing from 3rd to 2nd, guide the stick in three separate movements (down - left - down), don't try to grip it and force it straight to where you think second gear is.
For the question in the comment, accidentally grinding a shift won't do any serious damage so long as you don't grip the stick hard and try to force it to go where it doesn't want to. If you just "let go" of the stick when you hear the crunch, and try again, you will be very unlucky if you break anything.
With a modern gearbox it's not difficult to change gear without crunching and without using the clutch at all, if you get the rhythm of moving the stick and gas pedal (to match the engine revs) working together properly. You only need the clutch to start the car moving. But don't try practising that while you are still learning to drive. Leave it until you can change gear using the clutch without crunching 999 times out of 1000, and without having to think about what you are doing.