Correct spelling


Correct spelling, explanation: tandem is a noun that previously appeared in Latin. It was spelled with the suffix -em, so we cannot change its ending into -um, even though we associate it with Latin. In this case, the only correct form is the word tandem, not tandum.

Definition of tandem:
noun, a bicycle designed for two riders (one sits behind the other)
We borrowed a tandem so we could have some fun.
We rode a tandem to Madrid, it was a lot of fun!

Incorrect spelling


Incorrect spelling, explanation: if one’s not sure how to spell a specific word, they should become acquainted with its etymology. In this case, we can find out that the word tandem originates from Latin, so we mustn’t change its ending. The only correct form is tandem with the suffix -em, not tandum with the suffix -um.