1. جَاءَ أَحْمَدُ — Case of Aḥmad? Nominative subject.
2. مَرَرْتُ بِأَحْمَدَ — Why fatḥa? Genitive of a diptote.
3. ضَرَبَ زَيْدٌ — What is ḍaraba here? A past verb.
These test the key distinctions for the quiz. Explain the cause, not merely the final vowel.