Quote Originally Posted by ShakeyX View Post
So in response why is the example I posted; Концерт вчера вечером был плохой! why is плохой not плохим? as we have the verb Был and a subject + participle.
Hmmm... It is really very quirky. Sometimes Nominative is also possible and even preferable for был and будет. Let's see... Genitive is used to describe the property, while Nominative - to describe the equivalence.

X был/будет Y.

If you speak about a temporary property of a permanent object, then Genitive is preferable. Вчера огурец был зеленым, сегодня он желтый, завтра будет коричневым. (The same cucumber.) But Nominative here is also possible.
If you speak about a temporary object, which can be replaced with a similar one in different situation, Nominative is used. Вчера день был солнечный, сегодня - облачный, завтра будет туманный. No genitive is allowed because they are different days, not the same.

It is like the difference between Spanish verbs "ser" and "estar" if you know what they are.

So...

a) Концерт вчера был плохой. The concert of yesterday was bad. (But another concert will be good tomorrow.)
b) Концерт вчера был плохим. The concert was bad yesterday. (But tomorrow that concert will be good.)

The short form of adjective (плох) is used only as a predicate or a part of predicate and only instead of variant b).