duo

Latin declines only three of its cardinal numbers, and duo is one of them. Unus, duo and tres change their endings; everything from quattuor to centum sits there unaltered, and only from ducenti upwards do the numerals start declining again.

Duo carries a fossil of the dual number that Latin lost and Greek kept. Its forms are duo, duae, duo in the nominative; duorum, duarum, duorum in the genitive; duobus, duabus, duobus in the dative and ablative; and in the accusative duos or duo for the masculine, duas for the feminine, duo for the neuter. Ambo, "both", declines identically, and it is worth learning as a pair with this one.

Watch the mile. Mille passus is a thousand paces, one Roman mile; the plural is milia, a neuter noun taking a genitive, so two miles is duo milia passuum, "two thousands of paces". English took duo, dual, duet, double, duplicate, dubious and doubt (a mind in two), and two itself is a cognate rather than a borrowing.