«ظَنَنْتُ زَيْدًا قَائِمًا» presents Zayd’s standing under an attribution of conjecture, unlike the direct statement “Zayd is standing.” «عَلِمْتُ زَيْدًا قَائِمًا» instead attributes knowledge.
Such governing verbs may qualify not merely time but the speaker’s epistemic stance. Treating a verb of suspicion as equivalent to one of knowledge would erase a meaningful distinction.