Learn Farsi: این معدل تو است که اهم معیارها برای دانشگاه است. - It is your grade point average that is the most important criterion for the university.

ahamm is an Arabic elative adjective. Words in this 'af'al' pattern have a built-in comparative or superlative meaning. Because it shares the same root letters (h-m-m) as mohemm, it translates directly as 'more important' or 'most important'.

Why is there an Ezafe linking ahamm to me'yār-hā instead of putting the adjective after the noun?

While standard Farsi adjectives follow the noun, Arabic elatives acting as superlatives usually come before the noun and are linked with an Ezafe. The structure ahamm-e me'yār-hā literally means 'the most important of criteria' or 'the most important criterion'.

Why is the sentence phrased as in mo'addel-e to ast ke... instead of a simpler subject-verb structure?

This is a cleft sentence used for emphasis, exactly like in English. By separating it into 'It is your GPA that...', the speaker strongly emphasizes that the GPA is the deciding factor, not something else.

Why is the final verb ast singular when me'yār-hā (criteria) is plural?

There are two reasons here. First, inanimate plural subjects in Farsi often take a singular verb. Second, the true subject of that clause refers back to the singular GPA (mo'addel) from the first half of the sentence.