زَيْدٌ عَالِمٌ
Zayd is a learned person.
«عَالِمٌ» is a participial predicate presenting learning as a quality attributed to Zayd. The statement need not focus on an event at a particular moment.
زَيْدٌ يَتَعَلَّمُ
Zayd is learning / learns.
«يَتَعَلَّمُ» is a verbal predicate whose imperfect form may foreground an ongoing, repeated or renewed activity, depending on context.
Thubūt does not mean that every nominal predicate denotes an everlasting quality, nor does tajaddud mean that every imperfect indicates constant moment-by-moment change. Rather, a noun prominently characterizes, while a verb prominently presents an occurrence with its temporal aspect. The form of an overt predicate can therefore itself carry rhetorical significance.