Persian's indefinite article is ی (-i). It is added to the end of a noun phrase. If the preceding word ends in the vowel sounds â, o or u, the epenthetic consonant ی (-y-) is added between the word and the indefinite article.

a car
ماشین سفیدی
mâshin-e sefid-i
a white car
مشکلی هست
moshkel-i hast
There is a problem

The indefinite article should be written separately because it has a distinct grammatical role and is independent from its preceding word. English indefinite article a changes to an just as Persian indefinite article i changes to yi after certain vowels. However, in Persian's native alphabet, the indefinite article is written attached to the last element of the noun phrase. When Persian is written in Latin alphabet, the indefinite article, genitive marker, possessive adjectives, etc. are separated from their host word with a hyphen (-) for educational purposes.

Indefinite plural nouns

In Persian, -i can also mark plural nouns as indefinite. English does not have an exact equivalent for Persian's indefinite plural. It is translated with some, any or simply, a bare plural form.

some cars
ماشینهای سفیدی
mâshinhâ-ye sefid-i
some white cars

In the above examples, some is used to indicate indefinite plurals. In Persian, indefiniteness is marked with the article i, both in singular and plural forms.

در کمدم کتابی دارم
dar komod-am ketâb-i dâram
I have a book in my locker
در کمدم کتابهایی دارم
dar komod-am ketâbhâ-yi dâram
I have books in my locker

In the above examples, a bare plural form i.e. books indicates an indefinite plural.

More examples

some things
a little
چیزی در موردش نمی‌دانم
Chiz-i dar mowred-ash nemidânam
I do not know anything about it
کمی فارسی بلدم
Kam-i Fârsi balad-am
I know a little Persian
خانه‌ی بزرگی
khâne-ye bozorg-i
a big house
خانه‌ی بزرگتری
khâne-ye bozorgtar-i
a bigger house
روز خوبی داشته باشید!‏
Ruz-e khub-i dâshte bâshid!
Have a nice day!
چه روز زیبایی!‏
Che ruz-e zibâ-yi!
What a beautiful day!
در صورت‌حساب اشتباهی هست
Dar surat-hesâb eshtebâh-i hast
There is a mistake in the bill
پسر خوبی باش
Pesar-e khub-i bâsh
Be a good boy!
پسر بدی نیست
Pesar-e bad-i nist
He is not a bad boy

Special Structure

When a noun phrase comprises of a noun and one or more adjectives, the indefinite article can be directly added to the noun taking the place of genitive marker (e).

اتاق بزرگی
otâgh-e bozorg-i
a big room
اتاقی بزرگ
otâgh-i bozorg
a big room

This structure is mainly used when the adjective ends in the vowel -i:

مرد ایرانی‌ای
mard-e Irâni-i
an Iranian man
مردی ایرانی
mard-i Irâni
an Iranian man