Correct spelling, explanation: the word comes from Latin, in which it used to have the form secundus. 2nd appeared in English in the times of Middle English and it is the short form of the word second. As a short form, it is usually used in dates.

Definition of 2nd:
ordinal number – short of the word second, often used in written dates,
Let’s meet on the 2nd of May as we all have a day off then.

Collocations with 2nd:
Some typical collocations include:
2nd best – not as good as expected,
She is such a perfectionist – she is never satisfied with the 2nd best.
In the 1st/2nd place – an expression used to emphasize our opinion,
I don’t want to marry him – in the 1st place he is old and in the 2nd place he is poor.

Incorrect spelling, explanation: when we create ordinal numbers, we usually need the -th ending so we have such numbers as fifth, ninth, eleventh, and many others. As a consequence both learners and native users of English often assume that we do the same with the number 2, creating an incorrect form 2th. In the case of 2, however, the only correct form is 2nd, which is short of second.

Incorrect spelling

Incorrect spelling