Adjectives and nouns (substantives) are abstracted by the additions of the enclitic [-i].  An enclitic is a character that has no meaning on its own, until it is joined with another word.

good → goodness xub → xubi خوب ← خوبی
great → greatness bozorg → bozorgi بزرگ ← بزرگی
man → manhood mard → mardi مرد ← مردی
friend → friendship deust → deusti دوست ← دوستی

As you see, the Persian suffix of abstraction [-i] may be compared with the English suffixes of state or quality ([-ness], ‘happiness,’ [-ship] ‘friendship’), rank and domain ([-dom], ‘dukedom,’ ‘kingdom’), or state, condition, classification ([-hood], ‘fatherhood,’ ‘priesthood’):

happy → happiness xošhāl → xošhāli خوشحال ← خوشحالی
leader → leadership rahbar → rahbari رهبر ← رهبری
king → kingdom šah → šahi شاه ← شاهی
brother → brotherhood barādar → barādari برادر ← برادری

In addition, it also determines office, utilization:

fire fighter → fire department ātaš nešān → ātaš nešāni آتش نشان ← آتش نشانی
book seller → book store ketābforuš → ketābforuši کتابفروش ← کتابفروشی
chief of police (literally, ‘the protector of the city’)
→ police headquarters
šahrbān → šahrebāni شهربان ← شهربانی
sheriff → precinct, police department kalāntar → kalāntari کلانتر ← کلانتری
mayor → mayor’s office šahrdār → šahrdāri شهردار ← شهرداری
launderer → washing machine raxtšu → māšin-e raxtšui رختشو ← ماشین رختشویی
dishwasher → dishwashing machine zarfšu → māšin-e zarfšui ظرفشو ← ماشین ظرفشویی
to eat → tablespoon (literally, ‘eating spoon’) γazā xordan → qāšoq-e γazā xori غذا خوردن ← قاشق غذا خوری
fork → desert-fork (literally, ‘fruit-eating fork’) cangāl → cangāl-e mive xori چنگال ← چنگال میوه خوری

In dealing with the terminal vowels this ی keeps to the following orthographic and euphonic rules:

After the long [ā] and long [u] an euphonic [y] intervenes:

زیبا ← زیبایی
zibā → zibāi
‘beautiful, pretty’ → ‘beauty’

دانا ← دانایی
dānā → dānāi
‘learned; wise, sagacious’ → ‘wisdom; knowledge’

ظرفشو ← ماشین ظرفشویی
zarfšu → māšin-e zarfšui
‘dishwasher’ → ‘dishwashing machine’

زانو ← زانویی
zānu → zānui
‘knee’ → ‘knee-shaped, a goose-neck, sleeve-coupling, a siphon’

Normally adjectives with a final [i] are already a result of this construction, to begin with; such as., کار , noun ‘work, deed, job’ → کاری, adjective ‘effective; efficient, hard-working.’ Therefore they usually do not take this ”ی“ enclitic; instead they are used in a phrasal construction with the verb “to be”:

غنی ← غنی بودن
γani → γani budan
‘rich’ → ‘being rich, richness’

کاری ← کاری بودن
kāri → kāri budan
‘effective; efficient, hard-working’ → ‘effectiveness; efficiency’

Since in words of Persian origin the terminal [-e] (the silent /h, that is) represents the historical development an [-ag] suffix, in the process of abstraction the [-g] is reintroduced as a (sort of) means of euphony, which replaces the terminal [-e] and orthographically, the [-h]:

بچّه ← بچّگی
bacce → baccegi
‘child’ → ‘childhood; childishness’
خانه ← خانگی
xāne → xānegi
‘house, home’ → ‘belonging to the household’

An abstracted adjective could be used in the phrase به این/آن…ـی be in/ān abstracted adjective. i as the Persian counterpart for the English ‘so + adjective’ or ‘such a + adjective’ phrase:

به این زودی می روید؟
be in zudi miravid?
“Are you going so soon?”

مردی به این خوبی ندیده بودم
mardi be in xubi nadide budam
“I have never seen such a good man.”

An abstracted adjective could be used in the phrase به…ـی این/آن be abstr. adj. + –e in/ān as the Persian counterpart for the English “as…as…”:

این به خوبی آن نیست
in be xubi-ye ān nist
“This is not as good as that.”

آن ماشین به بزرگی این نیست
ān māšin be bozorgi-ye in nist
“That car is not as big as this (one).”