Ответ:

If I hadn’t been present at the lesson I should not have understood this difficult rules.--3 conditional (unreal past)

Объяснение:

Определите, какое из данных условно-придаточных предложений нереального типа.

Выберите один ответ:

If he reads fifty pages every day, his vocabulary increases greatly.

If I hadn’t been present at the lesson I should not have understood this difficult rules.

If they know it, they will take measures.