Why does حَضَرَ remain unchanged in إِنْ حَضَرَ الْمُعَلِّمُ نَبْدَأْ?

Conditional Sentences with إِذَا and لَوْ

• It is outside the condition.
• It is a noun.
• It is a built past verb; jussive does not change its form.
• It already lost its inflectional nun.