Correct spelling, explanation: the word fourteen is spelled with a u because it follows a consistent pattern in English spelling: number + -teen. There might be a little confusion, however, due to the fact, that we spell a different number, 40 (forty) without the u.
The term forty has its origins in Old English, specifically from the compound word feowertig which combines feower (four) with tig (a group of ten). Therefore, the spelling forty without the inclusion of the letter u has been in use for several centuries, dating back to at least the 16th century.
Fourteen on the other hand is a regular word, so the u remained and forteen is an incorrect form.

Definition of fourteen:
numeral, cardinal number between thirteen and fifteen
I have exactly fourteen marbles in my collection.
She is turning fourteen years old next week.
Fourteen students from our class went on a field trip to the science museum.

Incorrect spelling, explanation: the word forteen is incorrect because the correct word is fourteen. In the English language, the numbers from 13 to 19 are typically spelled with teen at the end, as in thirteen, fourteen, fifteen, and so on. This follows a consistent pattern in English numbering. The reason for this mistake might be the spelling of forty, where u is omitted. Forty comes from the Old English word feowertig and is an exception. The correct spelling of 14 is fourteen.

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling