الطَّالِبُ مُجْتَهِدٌ.
The student is diligent.
يَجْتَهِدُ الطَّالِبُ.
The student exerts effort.
The nominal construction lends itself to presenting a stable characterization, whereas the verbal construction can foreground action or renewed occurrence. This is a rhetorical tendency, not an inflexible rule: nominal clauses do not always prove eternity and verbal clauses do not always describe a single moment.
If the communicative aim is a continuing trait, the nominal assertion may be appropriate. A report of ongoing activity may prefer a verbal predicate. Thubūt and tajaddud depend on verbal aspect and discourse evidence together.
A past-to-imperfect shift may depict events vividly or habitually, but the imperfect does not automatically prove that a historical event is happening now.