The Demonstrative Adjective: When any of the demonstratives این in ‘this’ and آن ān ‘that, it’ accompany a noun, functioning as an adjective would, then it is called a demonstrative adjective. In Persian they are invariably singular, even when they modify a plural noun. Compare and contrast the following Persian and English sentences:

this book in ketāb این کتاب
that notebook ān daftar آن دفتر
these books in ketāb hā این کتاب ها
those notebooks ān daftar hā آن دفترها

The Demonstrative Pronoun: When the demonstrative replaces a noun (as a pronoun normally does), it is referred to as the demonstrative pronoun. In this case, they are pluralized when necessary, as any pronoun would:

This is a man. in mard ast .این مرد است
That is a woman. ān zan ast .آن زن است
These are men. in mard hastand .این ها مرد هستند
Those are women. ān hā zan hastand .آن ها زن هستند

A generic noun is invariably singular even when it is modified by a plural demonstrative pronoun:

.این زن است
in zan ast
“This is (a) woman.”

.این ها زن هستند
in hā zan hastand
“These are women.”

.این پسر دانش آموز است
in pesar dāneš āmuz ast
“This boy is (a) student.”

.آن پسرها دانش آموز هستند
ān pesar hā dāneš āmuz hastand

“Those boys are students.”