Most ordinal numbers end in "th" except for:
one ⇒ first (1st, 10th, 21st, 31st, 41st, 51st, 61st, 71st, 81st, 91st)
two ⇒ second (2nd, 12th, 22nd, 32nd, 42nd, 52nd, 62nd, 72nd, 82, 92nd)
three ⇒ third (3rd, 13th, 23rd, 33rd, 43rd, 53rd, 63rd, 73rd, 83rd, 93rd)