annus

annus, anni is a second declension masculine noun, entirely regular. What it really demands of a learner is the two time constructions. Time HOW LONG goes in the accusative with no preposition: tres annos mansit, he stayed for three years. Time WHEN goes in the ablative, again with no preposition: hoc anno, this year; anno superiore, last year. Getting that pair the wrong way round is the classic mistake.

Age is expressed by the same accusative of duration: quot annos natus es?, literally "how many years having been born are you?". Derivatives: annuus yearly, anniversarius, biennium and triennium for two and three year spans, and French an and année. One caution: anulus, a ring, is properly spelled with a single n and is a different word entirely; the spelling annulus is a medieval muddle with annus.