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:

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

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.