«وَأَصْلُ إِذَا الْجَزْمُ بِوُقُوعِهِ»
Idha basically presents occurrence with assurance and is often followed by a past-form verb.
إِذَا جَاءَ زَيْدٌ أَكْرَمْتُهُ
When Zayd comes, I shall honor him.
«جاء» has a past verbal form, but the conditional construction refers to a future event. The morphological tense of a word and the temporal interpretation of the complete clause must be distinguished.