Based on their structure, there are two classes of nouns in Persian: Simple Nouns, such as [tag resources Noun_1-09a]کِتاب[/tag] ketāb ‘book,’ and Compound Nouns, such as [tag resources ps Noun_1-09c]دانشمند[/tag] 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:

[tag resources ps Noun_1-04a].این کتاب است[/tag]
in ketāb ast
“This is a book.”

[tag resources Noun_1-05a].کتاب بهترین دوست انسان است[/tag]
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:

[tag resources Noun_1-18a]آقای احمدی[/tag]
āqāˈye ahmadi
‘Mr. Ahmadi’

[tag resources Noun_1-20a]آقای احمدی[/tag]
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:

[tag resources Noun_1-31a]یک مرد[/tag]
yek mard
‘a/one man’

[tag resources Noun_1-31b]دو مرد[/tag]
do mard
‘two men’

[tag resources Noun_1-31c]بیست مرد[/tag]
bist mard
‘twenty men’

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