I added a standard user, however, I'm unable to login. after inserting password, it takes me back to login screen.
Welcome to the Elementary OS forum.
Just to conform that the standard user account has the proper privileges to their own home directory please try these steps. Go to Applications -> Terminal (or Win+T).
Type the following command:
sudo ls -al "/home/userID"
Where userID is the name of the standard user account that you created. The listing provided will show you the first level of the standard user's home directory. You should see that userID and groupID are identical for everything except for the second line (that ends with ..), it should list the user as root and the group as root.
There are many additional levels to the home directory where privileges could be changed, but starting with the first layer is a good place to start.
Other solutions include
While in Terminal type the command:
Make sure that none of the Use% is near to 100%.
Finally, go to System Settings -> User Accounts
Then create another standard user by clicking on the '+' icon in the lower left corner of the window. Then try to log in as that user.
When I use my laptop on public networks, I always use my non-privileged account for security reasons.