debeo

Debeo, debere, debui, debitum, second conjugation. It is de plus habeo, "to have something from somebody", contracted, which is exactly what owing is. With a direct object it means owe; with an infinitive it becomes the workaday word for ought or must.

The tense catches everyone out. Latin puts the past marker on debeo where English puts it on the infinitive, so debebat hoc facere is "he ought to have done this", not "he ought to do this". Reading it the English way reverses the sense of a great deal of moral argument. The neuter participle debitum is a debt, and its plural is why the Lord's Prayer in English asks forgiveness for debts rather than trespasses. English debt, debit, due, duty and endeavour all come off this stem.