The action in subordinate clause refers to the past (model II) and the action in the principal clause is its consequence and refers to the present or future (model I).
If you had taken your medicine yesterday (II), you would be all right now (I).
Если бы ты принял свои таблетки вчера (II), ты бы сейчас был в порядке (I).The action in the subordinate clause is a constant quality and does not refer to any particular time (model I) and the action in the principal clause is its consequence, which refers to the past.
If she were not so short-sighted (I), she would not have taken me for her sister yesterday (II).
Если бы она не была такой недальновидной (I), она бы не отправила за своей сестрой вчера (II)If the acoustics in this hall were not so good, we should have heard nothing.
Если бы акустика в этом зале не была такой хорошей, мы бы ничего не услышали.