In «زَيْدٌ قَائِمٌ», both words have ḍamma, but their functions differ: Zayd is the topic, qāʾim the predicate.
In a basic nominal sentence, both are nominative. Identify the roles first, then give the case analysis.
«الطَّالِبُ مُجْتَهِدٌ»: «الطَّالِبُ» = nominative mubtadaʾ; «مُجْتَهِدٌ» = nominative khabar.