The rule of the using of short adjectives is not so easy in russian . But I want to point out the main rule: For example:how is it right to say about a boy with a weak health: Мальчик болЕН(short form) or Мальчик больНОЙ(full form)?=A boy is sick. Мальчик больНОЙ -- means that the boy is permanently sick,or has problems with his health permanently. Мальчик болЕН -- means that the boy is temporary sick,and the problems with his health is temporary. So, the difference is in the time description of the adjectives. When we want to say the full form of adjective while talking about the temporary situation, we should specify when the boy is sick or has problems,e.g. Мальчик СЕГОДНЯ(today) больНОЙ. In this case СЕГОДНЯ(today) point out that the situation is temporary.