neunzehn
nineteen
The cardinal number between eighteen and twenty, represented by the numeral 19.
Das Buch hat neunzehn Kapitel.
The book has nineteen chapters.

nineteen
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nineteen
The cardinal number between eighteen and twenty, represented by the numeral 19.
Das Buch hat neunzehn Kapitel.
The book has nineteen chapters.

The number 'neunzehn' (nineteen) follows the German system for numbers from 13 to 19: it is formed from the ones digit ('neun' = nine) and the word 'zehn' (ten).
Dreizehn, vierzehn, fünfzehn ... neunzehn, zwanzig.
Thirteen, fourteen, fifteen ... nineteen, twenty.
As a numeral before a noun, 'neunzehn' (nineteen) is not declined. It always keeps its base form.
Sie hat mit neunzehn Jahren ihr Studium begonnen.
She began her studies at nineteen.
To form the ordinal number, you add the ending '-te': 'der/die/das neunzehnte' (the nineteenth). For dates, the form 'am neunzehnten' (on the nineteenth) is used.
Wir feiern am neunzehnten Juni unseren Hochzeitstag.
We celebrate our wedding anniversary on the nineteenth of June.
nine
'neun' and 'neunzehn' are both numbers and have the same beginning. 'neun' means 9, while 'neunzehn' means 19. The risk of confusion comes from the fact that in 'neunzehn', you hear 'neun' first. But the ending '-zehn' shows that it is a number between 10 and 20. When you are talking about quantities, ages, or house numbers, this difference is important.

Tom hat im Kino neunzehn Euro für zwei Karten bezahlt.
Tom paid nineteen euros for two tickets at the movie theater.
Heute ist Aylin neun Jahre alt.
Aylin is nine years old today.