Rule 1: 1. يَا with a single noun: nominative without tanwīn
To call or address someone, Arabic puts يَا O! before the noun. If the noun stands alone, whether a proper name or a common noun used as a name for the person addressed, it takes a single ḍamma and loses its tanwīn: يَا مُحَمَّدُ, يَا وَلَدُ. The dual and the sound plural keep their nominative endings: يَا رَجُلَانِ, يَا مُسْلِمُونَ; a broken plural takes ḍamma: يَا أَوْلَادُ. Feminine names behave the same: يَا فَاطِمَةُ.
يَا حَارِثُ، سَاعِدْنِي فِي الْحَفْرِ.
yā ḥārithu, sāʿidnī fi l-ḥafri
Ḥārith, help me with the digging.
A proper name: ḍamma, no tanwīn.
يَا رَجُلُ، مَاذَا تَفْعَلُ هُنَا؟
yā rajulu, mādhā tafʿalu hunā?
Man, what are you doing here?
يَا أَوْلَادُ، لَا تَلْعَبُوا عِنْدَ الْبِئْرِ.
yā awlādu, lā talʿabū ʿinda l-biʾri
Boys, do not play by the well.
Broken plural with ḍamma.
يَا مُسْلِمُونَ، اِجْتَمِعُوا فِي الْمَسْجِدِ.
yā muslimūna, ijtamiʿū fi l-masjidi
Muslims, gather in the mosque.
Sound plural keeps ـُونَ.