(صفت + ترین = صفت برترین (صفت عالی
adjective + -tarin = superlative adjective

The superlative enclitic is ترین. The superlative adjective must invariably be followed by a noun directly, and without taking any preposition. While the subject of the sentence is specific (such as a hyponym), the noun that the tarin-phrase precedes normally is generic:

.گربه زیرک‌ترین حیوان است
gorbe ziraktarin heyvān ast
“Cats are the smartest of animals.”

.آبادان زیباترین شهر ایران است
ābādān zibātarin šahr-e irān ast
“Abadan is the most beautiful city of Iran.”

.شهرزاد بهترین شاگرد این کلاس است
šahrzād behtarin šāgerd-e in kelās ast
“Sheherzade is the best student in this class.”

A very specific “partitive” construction is formed by pluralizing the generic noun and connecting it to the preceding superlative adjective, using the ezāfe link:

.آبادان زیباترینِ شهرهای ایران است
ābādān zibā tarin-e šahr hyā-e irān ast
“Abadan is the most beautiful city of Iran.”
Literally, “Abadan is the most beautiful of the cities of Iran.”

A specific construction with the phrase بیش از پیش biš az piš (literally ‘more than before’) equates the English adverbial ‘more than ever’ and adjectival ‘more…than ever’:

.این دانش‌آموز بیش از پیش ترقّی کرده است
in dāneš āmuz biš az piš taraqqi karde ast
“This student has progressed more than ever.”

.آن شهر بیش از پیش آباد شده است
ān šahr biš az piš ābād šode ast
“That city has flourished more than ever.”

.شیرین بیش از پیش خوشحال است
širin biš az piš xošhāl ast
“Shirin is happier than ever.”

Since suffixes and postpositions cannot attach to the Silent /h/, neither tar nor tarin may be attached to it, even in the writings of those who opt to attach these suffixes to other words: بچّه‌تر bacce tar‘more childish’ and بچّه‌ترین bacce tarin ‘the most childish,’ never بچّهتر, بچّهترین; خسته‌تر xaste tar ‘more tired’ and خسته‌ترین xaste tarin ‘the most tired’, never خستهتر, خستهترین.

Even if we opt to connect the comparative and superlative enclitics, we better write them separately if the adjective ends in a terminal ـت –t: سخت‌تر ‘harder,’ not سختتر; درشت‌تر ‘larger,’ not درشتتر.

The superlatives of خوب and زیاد / بسیار are بهترین and بیشترین. Occasionally in the colloquial dialect one would hear خوبترین and زیادترین.

While normally the preposition for the second term of comparison is از, a different construction is used with تا in vary specific situations, as follows:

If we opt to place the second term of the comparison after the verb:

.این بچهّ بیشتر غذا خورد تا شما
in bacce bištar γazā xord tā šomā
“This child ate more than you (did).”

.هفت بیشتر است تا چهار
haft bištar ast tā cahār
“Seven is more than four.”

.برای این کار کامران بهتر است تا محمّد
barā-ye in kār kāmrān behtar ast tā mohammad
“Kamran is better for this job than Mohammad.”

When the second term of the comparison is the object of the sentence—direct or indirect:

.با او بیشتر حرف زدم تا با تو
bā u bištar harf zadam tā bā to
“I talked with him/her more than I did with you.”

.کامبیز را بیشتر دوست داشت تا کورش را
kāmbiz rā bištar dust dāšt tā kuroš rā
“He/she liked/loved Kambiz more than Koorosh.”
“He/she liked/loved Kambiz more than he/she loved Koorosh.”

When the quality in something is less than expected, then a lower quality material (as a noun) follows the comparative adjective and the expected quality follows the :

.این بیشتر آب است تا شراب
in bištar āb ast tā šarāb
“This is more [like] water than wine.”
“This is more water than [it is] wine.”

.این بیشتر قاطر است تا اسب
in bištar qāter ast tā asb
“This is more a mule than a horse.”