Correct spelling, explanation: this is the correct form of thirty-first because the end of this ordinal number is -st and so should be the ending of the numeral form. This is basically the rule for all ordinal numbers: third is 3rd, thirty-first is 31st, and fifty-fourth is 54th. Therefore
31th is incorrect.
Definition of 31st:
numeral, the ordinal number between 30th and 32nd
On Jack’s 31st birthday we had a huge party.
It’s crazy, it’s your 31st job!
Incorrect spelling, explanation:
31th is incorrect because the word thirty-first doesn’t end with th but with st. 31st is thus the correct form. Ending -st is added to the number first and any other ordinal number ending with first, so 31st is the correct form.
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Perfect information. I will be dealing with some of these words in my essay.