It all matters on pronouncation. You know the drill for 1 digit numbers,
And take a look at ’21”s last digit. It’s one, so it becomes first. But, ’11”s pronouncation is eleven, it does not end with the word one. Therefore, it is 11th. Here are some examples to make you understand.
28th birthday = twenty EIGHT, eight uses th so 28th.
23rd of November = twenty THREE, three uses rd so 23rd.
Also, remember that all numbers from 10-20 uses th because they do not pronounce the last digit as how they would in a single digit.Examples
15th = FIFTEEN, doesn’t end with five, so th.
13th = THIRTEEN, doesn’t end with three, so th.