What is a common skill?
Each class will have 2 common skills. Common skills can be used by all classes.
How to use skills? How to use hot keys?
Right-click with the mouse to use the skill (requires target selection); the icons of the skills can be freely dragged in the middle bottom of the game screen for convenient and quick use or activation.
How to obtain new primary skills?
By leveling up your class, you can acquire primary skills. The higher the level, the more skills you can learn.