Learn Farsi: خوبی و زیبایی در طبیعت انسان است.

It is formed by adding the suffix -i (ی) to the adjective khub (good). This turns the adjective into an abstract noun, working just like the -ness ending in the English word goodness.

Why is there an extra letter in zibâyi (beauty) compared to khubi?

The base adjective here is zibâ (beautiful), which ends in the long vowel â. In Farsi, when you add the -i suffix to a word ending in a vowel, a bridge letter -y- (ی) is inserted to separate the vowels and make it easier to pronounce.

The subject is goodness and beauty (two things). Why does the sentence use the singular verb ast (is)?

In Farsi, if the subject of a sentence consists of inanimate objects or abstract concepts (like goodness and beauty), the verb usually takes the singular form ast (is). Plural verbs are strictly required only when the plural subject is human.

How do the words tabiat (nature) and ensân (human) connect in this sentence?

They are connected by the ezafe linking vowel. Even though it is not written out as a full letter in the Farsi alphabet, you pronounce a short e at the end of tabiat. This creates the phrase tabiat-e ensân, meaning the nature of humans or human nature.