Based on their structure, there are two classes of nouns in Persian: Simple Nouns, such as کِتاب ketāb ‘book,’ and Compound Nouns, such as دانشمند dānešmaˈnd ‘an erudite, a knowledgeable person.’

In dealing with the nouns, Single or Compound, the following points should be kept in mind.

To begin, if a noun is not modified by an adjective or a number, it is invariably single and absolute:

.این کتاب است
in ketāb ast
“This is a book.”

.کتاب بهترین دوست انسان است
ketāb behtarin dust-e ensān ast
“Books are a man’s best friend.” (literally, “Book is man’s best friend.”)

In Persian the stress is always on the last syllable of the noun, whether it is of Persian or non-Persian origin (i.e., a loanword); thus:

book ketˈāb کِتاب
child bacˈce بَچِّه
telephone teleˈfon تِلفن
Anthony āntuˈni آنتونی
Muhammad mohamˈmad مُحمّد
mountains (range) kuhestˈān کوهِستان
computer kāmpuˈter کامپیوتر

In the vocative, the stress moves to the first syllable of the noun; such as:

hey boy/girl/kid bacˈce! بچّه
O erudite one! ey dāˈnešmand ای دانشمند
O God! (ey) xoˈ ای خدا
O Mr. Ahmadi! āˈ-ye ahmadi! آقای احمدی

Compare and contrast the following:

آقای احمدی
āqāˈye ahmadi
‘Mr. Ahmadi’

آقای احمدی
āˈqā-ye ahmadi!
“O, Mr. Ahmadi!”

With two stresses: one on the last syllable of āqā, and the other on the last syllable of ahmadi.

A noun modified by a number is never pluralized; that is to say, it is invariably singular, for example:

یک مرد
yek mard
‘a/one man’

دو مرد
do mard
‘two men’

بیست مرد
bist mard
‘twenty men’

As we can see, the noun (that is, مرد mard ‘man’) stays constant and in singular.