Incorrect spelling, explanation: this spelling can be a common mistake because most ordinal numbers are created by adding the suffix -th to a number (for example 40th, 56th, and 94th). Number one, two, and three are the exceptions. When we want to spell an ordinal number, which ends with the number one, we must add the -st suffix to it. Therefore, the correct form is 21st, and 21th is incorrect.

Correct spelling, explanation: 21st is an ordinal number and its full form is spelled twenty-first. It appears between twentieth and twenty-second. When an ordinal number ends with the number one, we add the irregular suffix -st to the number (except for 11, which is spelled 11th). That’s why the correct form is 21st, not 21th.

Definition of 21st:
numeral, coming next after the twentieth in position
It’s my 21st time in the church – it’s almost hard to believe!
My grandmother said she was 21st in line.

Collocations with 21st:
Some most commonly used collocations include: 21st century, 21st birthday, and 21st day of a month.
The 21st century is really complicated.
Did you buy your son a present? It’s his 21st birthday!
I’m getting married on 21st August.

Incorrect spelling

Incorrect spelling