It is not a verb form. It is just a short form. Both forms can be used as predicates but have different meanings. For example, "ты болен" and "ты больной" both mean "you are sick", but in the first case it means a temporary state. Another example, "эта юбка коротка" and "эта юбка короткая" both mean "this dress is short", but in the first case it has an implication of being short relative to something. For example, you would use first form to mean that this dress is short for you.

Hmm that's also possible. Only those who have studied the history of Esperanto can give us a definite answer, I suppose.

Wait, where's the verb in the first form?

красива is predicate (or сказуемое)