How to inherit from other class in c#?

Hey, I am literally untouched by OOP and also by c# so this question will be rather trivial.

Lets say I want to have 100+ of skills:
classes Fireball, Frostbolt, Heal, … which represent my ingame skills and will be attached to GOs.

I want them all to have some common variables and functions: SkillStart, SkillEnd, SkillInterupt, …

Normally, I would create common abstract ancestor class for them which would implement those common functions.

But they already inherit from MonoBehaviour so they cannot inherit from another class, right?

Also I have no idea how monobehaviour works behind the scenes so I dont want to do something really dumb.

Have your skill base class inherit from MonoBehaviour:

public abstract class SkillBase : MonoBehaviour
    //put the common variables/functions here

public class Fireball : SkillBase 
    //fireball specific stuff here