After entering the password for ubuntu login, the system still returns to the login interface.

After entering the password for ubuntu login, I still returned to the login interface. Today, I restarted the system and logged on to the system. After I entered the password, the screen turned black, but I still returned to the login interface. I checked the Web site for a long time and restarted www.2cto.com. press shift to go To the Grub interface, select recovery so even if the root user goes in, he still does not have the write permission and then executes mount-o rw, you can obtain the read and write permissions by using remount/. You can view the permissions in the/home/username directory. in the xsession-errors file, it will tell you the cause of startup failure. the code with syntax errors added to the profile file causes the first phenomenon to be solved. Tks god!