primus

Primus, prima, primum is a first and second declension adjective, and it is a superlative in form with no positive of its own. Its comparative partner is prior, "the earlier of two", so Latin distinguishes the first of two from the first of many in a way English does not bother to.

Two forms do adverbial duty and are not the same. The neuter accusative primum means "first, for the first time" and orders a list. The ablative primo means "at first, in the beginning" and sets up a contrast with what happened later, primo ... deinde. In primis, sometimes written imprimis, means "chiefly, above all".

Watch the "part of" idiom, which is common with adjectives of position: prima nox is not "the first night" but the first part of the night, and prima luce is at the first of the light, so at dawn. It gives prime, primary, primitive, premier and prince (princeps, "taking first place").