Correct spelling

41st

Correct spelling, explanation: to all numbers ending with number one we add suffix -st when creating ordinal form (81st, 101st, 23 etc.) This is why here the correct form is 41st. Suffix -th is a usual one for ordinal numbers, but there are exceptions and number 1 is one of them, therefore 41th is a wrong form.

Definition of 41st:
numeral, ordinal number between 40th and 42nd
My grandparents have their 41st wedding anniversary next week.
Jack was 41st in the New York marathon, so it was a huge success.


Incorrect spelling

41th

Incorrect spelling, explanation: 41th is a wrong form, because suffix -th is not the one used to create ordinal form for number 1. The correct suffix is -st, so the proper way to write this ordinal number is 41st.

Incorrect spelling

41nd

Incorrect spelling

Incorrect spelling

41rd

Incorrect spelling