When أَحْمَدُ is a person’s name, it has proper-name status, and its أَفْعَل shape resembles a verbal pattern.
جَاءَ أَحْمَدُ — Aḥmad came; nominative subject.
رَأَيْتُ أَحْمَدَ — I saw Aḥmad; accusative object.
مَرَرْتُ بِأَحْمَدَ — I passed by Aḥmad; genitive position, fatḥa ending.
The second cause here is the verb-like pattern, not adjectival use.
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Definiteness and Proper-Name Status in Diptote Rules
When أَحْمَدُ is a person’s name, it has proper-name status, and its أَفْعَل shape resembles a verbal pattern.
جَاءَ أَحْمَدُ — Aḥmad came; nominative subject.
رَأَيْتُ أَحْمَدَ — I saw Aḥmad; accusative object.
مَرَرْتُ بِأَحْمَدَ — I passed by Aḥmad; genitive position, fatḥa ending.
The second cause here is the verb-like pattern, not adjectival use.