Welcome. If this is your first real look at Vietnamese grammar, start here. Nouns are where the language feels most different from English, and once these click, half the early confusion disappears.
The good news first: Vietnamese nouns don't change. The word sách (book) is sách whether you mean one book, two books, the book on your desk, or all books ever printed. No plural -s, no possessive 's, no him/his/he shape-shifting. Same word, every time.