the vast majority also don’t speak anything like that, they speak local dialects which vary widely.
but the one thing in common is Modern Standard Arabic, which does involve casings like nunation, so it is the only right way to teach ‘Arabic’ unless one wants to learn a specific localized dialect.
both have nunations, but different casings.
Possession when indefinite (mudhaaf wa mudhaaf ilayh) have a nunation with a kasra. When it is definite, the nunation drops. كتابُ طالبٍ
non definitive equational sentences (sentences that start with a noun and have no verb - or jumlatun 2ismiyyah - mubtada2 wa khabar) have a nunation with a dhamma, unless they are definitive the nunation drops.
These rules as I stated are not for any dialect though, they are MSA. Hope it helps :)
it is a name but it is also a native Arabic word (it means along the line of 'majestic' in feminine adjective) so it is subject to casing (in this case: nunation) depending on its position in the sentence.
It also ends with a taa2 marbuutah which is a letter that is either an h or a t depending on whether the word is in mid or end sentence.
So if pronounced alone it would be saamyah
If in this form it is saamyatun dhakiyyah
no it is not the plural, it is a nunation which is a form of grammatical casing - it is the same word. If a nunation is applied on a taa2 marbuutah, it changes in pronunciation from h to t then the nunation follows.
if the word is by itself or at the end of a sentence it is pronounced saamyah
if the word is followed by a non-definitive adjective as in this case it would be samyaatun dhakiyyah
ill add that 'saamyatu dhakiyyah' is also correct given that it is also a name...