mens

Mens, mentis is third declension feminine and an i-stem, so the genitive plural is mentium. The stem is ment-, and the nominative is that stem plus -s with the t lost in the spelling, exactly as in gens and mors.

The ablative mente has a long afterlife. Latin said bona mente, "with a good mind", to describe how something was done, and that phrase hardened into the Romance adverb ending: Spanish rápidamente and French rapidement are both this noun, fossilised. Worth knowing before you meet it in a grammar. The distinction to hold is against animus: mens is intellect, judgement and intention, animus is spirit, will and temper. They overlap and are not swappable. Family: mentio, memini, comminiscor, demens, vehemens. English mental, demented, reminiscence and, at one remove, mention.