I have a new tutorial for LG device. And this one is for LG Optimus G E971 this also work in E973 variant.
Hard resetting / factory resetting your phone will solve the following issues:
1. If you forgot your pattern lock 2. If you forgot your gmail account 3. If you forgot your password 4. Apps that automatically force closing 5. Stuck in LG Logo (sometimes does not work if the firmware is totally damage)
NOTE: Performing hard reset will erase your data.
To hard reset:
1. Turn off your phone. 2. Press and Hold VOLUME DOWN + Power button.Then immediately insert the USB cable. Your usb cable must be plugged in a power on laptop or pc. You should see a text with a black ground in it.
3. Press Power Button two times to Factory Reset your phone. 4. Reboot your phone.
Note: If performing hard reset did not succeed, you need to wipe the cache first in the android system recovery before performing wipe / factory reset.
I hope this tutorial helps you.. If you have any question just drop a comment.