necesse

Necesse is indeclinable and always neuter. There is no necessis, no necessem, no plural - it is a fossil, and it lives almost entirely in the phrase necesse est. That phrase takes either an accusative with an infinitive (necesse est te venire) or, increasingly in later and ecclesiastical Latin, an ut clause with the subjunctive.

The construction worth drilling is the person under the obligation. Latin puts them in the dative: mihi necesse est abire, "I have to leave". English frames the same thought with the person as subject, so the shape has to be rebuilt every time. The living adjective in the family is necessarius, and the abstract noun is necessitas; necesse itself never grew any further forms.