Incorrect spelling, explanation: the correct written form of this number is nineteen as we don’t drop the e from the end when the suffix starts with a consonant. The suffix -teen starts with t so e in nine remains. Pronunciation also points to spelling nineteen. Thus, ninteen is the wrong form.

Correct spelling, explanation: nineteen is the correct form because we do not drop the final e with suffixes unless the suffix starts with a vowel. Here, the suffix -teen starts with a consonant, so we do not omit the vowel e, and this way we get the form nineteen. Skipping the e, so spelling ninteen, is incorrect.

Definition of nineteen:
numeral, the cardinal number between eighteen and twenty
His aunt had a huge house with nineteen cats.
– How much is this shirt? – It’s nineteen pounds.