Mansub does not mean that every noun must display the same ending.
Fathah in these singular nouns: زَيْدًا، مُحَمَّدًا، الْإِسْلَامَ، الصِّحَّةَ. Each is the ism of one of the six particles.
Ya in the sound masculine plural: الْكَافِرِينَ. It is also mansub, but ya is its sign of nasb.
First identify the grammatical reason; then read the case sign appropriate to the noun's form.
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Introduction to Mansubat and the Ism of Inna
Mansub does not mean that every noun must display the same ending.
Fathah in these singular nouns: زَيْدًا، مُحَمَّدًا، الْإِسْلَامَ، الصِّحَّةَ. Each is the ism of one of the six particles.
Ya in the sound masculine plural: الْكَافِرِينَ. It is also mansub, but ya is its sign of nasb.
First identify the grammatical reason; then read the case sign appropriate to the noun's form.