In English, the definite article is used to indicate a noun that is obvious or identifiable to the listener. One of the trickiest parts in learning a foreign language is to know when a definite article is used and when it is not. Persian is quite easy and straightforward in this respect: there is no definite article. Nouns are definite by default.

ماشین در پارکینگ است
Mâshin dar pârking ast
The car is in the garage

In the above sentence, it is known to the listener which car and which parking the speaker means. English uses "the" but in Persian the lack of an article indicates that the noun is definite.

آسمان صاف بود
Âsemân sâf bud
The sky was clear
خورشید می‌درخشید
Khorshid miderakhshid
The sun was shining

In the above sentences, sky and sun are obvious to the listener. English uses "the" but in Persian a bare noun is definite.

دارد گیتار یاد می‌گیرد
Dârad gitâr yâd migirad
She is learning the guitar

In the above sentence, guitar is a generic noun which represents a whole group: what she is learning is the guitar and not the violin, for example. English often uses "the" for generic nouns but in Persian, generic nouns are intrinsically definite.

از مار می‌ترسد
Az mâr mitarsad
He is afraid of snakes

In the above sentence, snakes is again a generic noun: he is afraid of snakes and not spades, for example. Here, English is using a plural noun but in Persian, generic nouns are usually expressed in singular form and are automatically definite.

ورزش برای سلامتی خوب است
Varzesh barâ-ye salâmati khub ast
Exercise is good for health

In the above sentence, English is using no articles for exercise and health. This can give you an idea of how definiteness works in Persian: bare nouns are definite.

ماشین از موتور امن‌تر است
Mâshin az motor amntar ast
Cars are safer than motorcycles

The above sentence is another example of use for generic nouns: cars and motorcycles represent all members of their class. Persian typically uses singular nouns to express a generic concept.

ماشین دارید؟
Mâshin dârid?
Do you have a car?
ماشین ندارم
Mâshin nadâram?
I do not have a car

In the above sentences, English is using the indefinite article a to refer to generic nouns. Persian is using a bare singular noun, just as in the previous examples.

مادرش مهندس است
Mâdar-ash mohandes ast
His/Her mother is an engineer

The above sentence is another example for when English uses the indefinite article to refer to a general type of person or thing (a generic noun). Persian does not use indefinite article to indicate generic nouns.

More examples

موز دوست دارد
Mowz dust dârad
He likes bananas
کجا می‌توانم بلیط بخرم؟
Kojâ mitavânam belit bekharam
Where can I buy tickets?
پدرش وکیل است
Pedar-ash vakil ast
His/Her father is a lawyer
ساندویچ می‌خواهی؟
Sândevich mikhwâhi?
Would you like a sandwich?
پزشکی می‌خوانم
Pezeshki mikhwânam
I study medicine
سفر چقدر طول می‌کشد؟
Safar cheghadr tul mikeshad?
How long does the trip take?

Demonstrative adjectives

Definite articles usually stem from demonstrative adjectives. For example, English the derives from an Old English demonstrative. In Persian, demonstrative adjectives can be used to mark definiteness explicitly:

این همان خانه است
In hamân khâne ast
This is the house

Accusative case

The accusative postposition را () can make generic nouns specific and identifiable to the listener. Compare:

کتاب خریدم
ketâb kharidam
I bought books
(What I bought was "book" and not "pen", for example)
کتاب را خریدم
ketâb kharidam
I bought the book
(A specific "book" that is identifiable to the listener)
کتابها را خریدم
ketâb kharidam
I bought the books
(Specific "books" that are identifiable to the listener)