Why is the idiomatic translation of إِذَا حَضَرَ الْمُعَلِّمُ بَدَأَ الدَّرْسُ not forced into the past?

Conditional Sentences with إِذَا and لَوْ

• The verbs are nouns.
• إِذَا can give present/future meaning with past-form verbs.
• The sentence has no condition.
• Every past verb means future.