15.2.5 Effective base class documentation is really confusing

I'm having a hard time understanding the effective base class spec in various ways:
- "For each constraint of T that is a struct-type" - how is that feasible? Ditto enum types and delegate types.
- "If the type parameter is a method type parameter whose constrains are inherited from the base method the effective base class is calculated after type substitution" - substitution of what? An example would really help here
- "A type parameter is known to be a reference type if it has the reference type constraint or its effective base class is not object or System.ValueType" - what about System.Enum, which is included in the definition of R earlier on...