Incorrect spelling, explanation: this form is incorrect because this ordinal number full-word form is thirty-first. Thus a suffix for the numeral form is -st as in first, not -th, so 31th is an incorrect form and 31st is the correct one. 31st is correct due to the thirty-first form.
Correct spelling, explanation: 31st is the correct form, due to the nature of the ordinal number 1st (first). As an irregular ordinal number first has got a different suffix, based on the ending of the written (full-word) form (st). Thus this suffix is added to all numbers ending with first. th on the other hand is added to those ordinal numbers, which end with this sound in their full-word form. This means that 31th is incorrect and 31st is the correct form.
Definition of 31st:
numeral, the ordinal number between 30th and 32nd
Tomorrow’s my 31st birthday!
This is your 31st pair of shoes, I think you have enough of them!
Incorrect spelling
Incorrect spelling
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Seems correct.
Well, I wasn’t taught this way. Thanks for the knowledge.