Ask four questions when parsing a noun:
1. What is its function—subject, object, or noun governed by a preposition?
2. Is its case nominative, accusative, or genitive?
3. Is it munṣarif or ghayr munṣarif?
4. Which case marker applies, and does tanwīn occur?
In «بِأَحْمَدَ», bi- governs the genitive, while Aḥmad's diptote status explains the fatḥa marker.