zehn
ten
The cardinal number 10; the number after nine and before eleven.
Ein Jahrzehnt dauert zehn Jahre.
A decade lasts ten years.

ten
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ten
The cardinal number 10; the number after nine and before eleven.
Ein Jahrzehnt dauert zehn Jahre.
A decade lasts ten years.

When 'zehn' (ten) is used as a number word (adjective), it is written in lowercase. When it is used as a noun for the digit itself, it is capitalized and has the article 'die'.
Ich habe zehn Euro in meiner Tasche.
I have ten euros in my pocket.
'Zehn' (ten) is the basis for the numbers from 13 to 19 (e.g. 'dreizehn' — thirteen, 'vierzehn' — fourteen) and for the tens numbers (e.g. 'zwanzig' — twenty, 'dreißig' — thirty). It is also part of words like 'Jahrzehnt' (decade).
Die Konferenz findet in dreizehn Tagen statt.
The conference takes place in thirteen days.
The phrase 'eine glatte Zehn' (a perfect ten) is a fixed expression. It is used to express that something is perfect or deserves the highest rating.
Deine Präsentation war super, von mir bekommst du eine glatte Zehn!
Your presentation was great; from me you get a perfect ten!
tenth
'zehn' and 'zehnt' look similar and both belong to the number 10. You use 'zehn' for counting: 'zehn Euro' (ten euros), 'zehn Minuten' (ten minutes), 'zehn Personen' (ten people). You use 'zehnt' for an order or sequence, for example 'der zehnte Tag' (the tenth day) or 'die zehnte Frage' (the tenth question). There is also the form 'zu zehnt' when a group of ten people does something together. So the risk of confusion is that 'zehn' states the quantity, while 'zehnt' usually indicates the position in a sequence.

Beim Arzt wartet die Verkäuferin schon seit zehn Minuten.
At the doctor's office, the saleswoman has already been waiting for ten minutes.
Sie ist heute die zehnte Kundin im Büro.
She is the tenth customer in the office today.