A copula is a word that combines the subject and its explanation. The verb “be” is the English copula. Persian has two copulas: ast and hast. hast is used for existential interpretations, and has a negative counterpart nist. ast is frequently reduced to the vowel [-e].

The long copulas are also used for emphasis:

کلاس فارسی شما سخت هست یا نه؟
kelās-e fārsi-ye šomā saxt hast yā na?
“(Well, I am not clear) Is your Persian class difficult, or is it not?”

Compare that with:

کلاس فارسی شما سخت است یا نه؟
kelās-e fārsi-ye šomā saxt ast yā na?
“Is your Persian class difficult, or not?”

In addition, they are used whenever in English “is/are there” is used:

آیا در برکلی آپارتمانِ ارزان هست؟
(āyā) dar berkli āpārtemān-e arzān hast?
“Are there any inexpensive apartments in Berkeley?”

Compare that with:

آیا در برکلی آپارتمانِ ارزان است؟
(āyā) dar berkli āpārtemān arzān ast?
“Are apartments inexpensive in Berkeley?”

Some examples:

من دانشجو هستم
man dānešju hastam
“I am a (college/university) student.”

او ایرانی نیست
u irāni nist
“He/she is not Iranian.”

تو کی هستی؟
to ki hasti?
“Who are you?”

در دانشگاه ما کرد هست؟
dar dānešgāh-e mā kord hast?
“Are there any Kurds in our university?”

آنها امروز در شیراز هستند
ānhā emruz dar širāz hastand
“They are in Shiraz today.”

!امروز هوا خوب نیست
emruz havā xub nist
“Today the weather is not good!”

آیا نزدیک خانه‌ی شما رستوران چینی هست؟
āyā nazdik-e xāne-ye šomā resturān-e cini hast?
“Are there any Chinese restaurants near your house?”

امروز خیلی خسته هستم
emruz xeyli xaste hastam
“I am very tired today.”

To reiterate, the Long Copulas in Persian originated from the now abandoned هستن hasten ‘to exist’. In modern Persian this verb only exists in the conjugated form, and the infinitival form has been abandoned entirely. It may be replaced in the sentence وجود داشتن.

The affirmative form is conjugated as follows:

The negative is as follows:

hast has an emphatic roll, in which case it takes the stress.

With the exception of the third person singular, this verb may replace the personal enclitic copulas. They are particularly useful for euphonic purposes, when we are dealing with terminal vowels before the copulas, especially in the case of the clash of adjacent like sounds:

تو ایرانی‌یی/ای < تو ایرانی هستی؟
to iran-yi< to irani hasti? (singular)
“Are you Iranian?”
شما ایرانی‌یید/اید < شما ایرانی هستید؟
šomā iran-yid > šomā irani hastid? (plural)
“Are you Iranian?”
پس آن‌ها کجا‌اند/کجایند؟ < پس آن‌ها کجا هستند؟
pas ānhā kojā-and/yand < pas ānhā kojā hastand?
“So where are they?”
آیا شما آمریکایی‌یید؟ < آیا شما آمریکایی هستید؟
āyā šomā āmrikā-yid < āyā šomā āmrikāi hastid?
“Are you American?”

In the case of the third person singular, however, we cannot replace است with هست, unless it has an emphatic roll, in which case it takes the stress.

Compare and contrast the following sentences:

او دانشجوی خوبی است
u dānešju-ye xu’bi ast
“She/he is a good student.”

(او دانشجوی خوبیست contracted)
او دانشجوی خوبی هست
u dānešju-ye xubi ha’st
“She/he (really) is a good student.”

As in the second English sentence is takes the stress from the adjective “good,” so does the third person singular هست in Persian take away the stress from the adjective خوب; whereas in the case of all persons other than the third person singular, if the stress stays on the adjective, it means ‘am, are’ (as in من خوبم and من خوب هستم man xu’b hastam ‘I am fine’), but if the stress is moved to hast, it is emphatic; as in, من خوب هستم man xub ha’stam ‘I (really) am fine [not to worry!]’.

Therefore, if the verb is not emphatic, then we would get:

هستم٬ هستی٬ است٬ هستیم٬ هستید٬ هستند

And if the verb is emphatic, or if it means “to exist,” then we have:

هستم٬ هستی٬ هست٬ هستیم٬ هستید٬ هستند

hast is also an affirmative reply to an otherwise negative connotation/suggestion. Compare and contrast the following sentences:

حسن دانشجوی خوبی نیست
hasan dānešju-ye xubi nist
“Hasan is not a good student.”
!چرا٬ هست
cerā, ha’st
‘Why of course he is; but of course he is.”

در یخچال غذا نیست؟
dar yaxcāl γazā nist?
“Isn’t there any food in the refrigerator?”
!چرا٬( در یخچال غذا) هست
cerā, dar yaxcāl γazā ha’st
“Yes, there is (food in the fridge).”