Unter vs. unten
The German words unter and unten both relate to the English concepts of under or below, but they are used very differently. The fundamental difference is their grammatical function: unter is a preposition, while unten is an adverb.
Unter
A1Unter is a preposition, which means it describes the relationship between two things and must be followed by a noun or pronoun. It answers the question where something is in relation to something else. It can mean under, below, beneath, or among.
Die Katze schläft unter dem Tisch.
(The cat is sleeping under the table.)
Der Taucher schwimmt unter der Wasseroberfläche.
(The diver is swimming below the water's surface.)
Er legte den Brief unter das Buch.
(He put the letter under the book.)
Die Temperatur liegt unter dem Gefrierpunkt.
(The temperature is below freezing.)
Unter diesen Umständen können wir nicht arbeiten.
(We cannot work under these circumstances.)
Es gab einen Spion unter den Soldaten.
(There was a spy among the soldiers.)
Unten
A1Unten is an adverb of place. It specifies a location (down below, at the bottom, downstairs) and does not require an object to follow it. It stands on its own to answer the question Where?.
Meine Wohnung ist ganz oben, seine ist unten.
(My apartment is at the very top, his is downstairs.)
Ich warte unten am Eingang auf dich.
(I am waiting for you down at the entrance.)
Bitte unterschreiben Sie unten auf dem Formular.
(Please sign at the bottom of the form.)
Wo sind die Kinder? Sie spielen unten im Garten.
(Where are the children? They are playing down in the garden.)
Der Keller ist hier unten.
(The basement is down here.)
Im Text weiter unten finden Sie weitere Informationen.
(You will find more information further down in the text.)
Summary
In short, use the preposition unter when you specify what something is under (e.g., unter dem Baum - under the tree). Use the adverb unten when you are simply indicating a general lower location like downstairs or at the bottom without connecting it directly to another noun (e.g., er ist unten - he is downstairs). Unter needs a noun; unten stands alone.