It's 3rd. 1st. 2nd. 3rd. From 4th onward until you come back to 21st, 22nd, 23rd it's a th. Then keep doing that for every number that does not end in 1, 2, or 3, excluding 11, 12 and 13, which all get the th i.e. 11th, 12th, 13th. English is weird, I know. Alternatively you can just spell the...