zusammengesetzt
composed
Consisting of several individual parts, components, or elements.
Ein Molekül ist aus verschiedenen Atomen zusammengesetzt.
A molecule is composed of different atoms.

composed / compound
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composed
Consisting of several individual parts, components, or elements.
Ein Molekül ist aus verschiedenen Atomen zusammengesetzt.
A molecule is composed of different atoms.

'Zusammengesetzt' is the past participle of the separable verb 'zusammensetzen' (to put together; zusammen + setzen). It is used like an adjective and literally means 'something that has been put together.'
Der Schrank muss noch zusammengesetzt werden.
The cabinet still has to be assembled.
The meaning depends strongly on the context. In connection with materials or systems, it means 'consisting of.' In a linguistic context, it describes a word. When it describes a person, it means 'calm and collected.'
Ein Team ist aus verschiedenen Persönlichkeiten zusammengesetzt.
A team is composed of different personalities.
In the meaning 'consisting of,' the adjective is almost always used with the preposition 'aus' and the dative. The structure is: [something] ist aus [parts] zusammengesetzt.
Die Prüfung ist aus einem mündlichen und einem schriftlichen Teil zusammengesetzt.
The exam is composed of an oral and a written part.
When 'zusammengesetzt' describes a person who is calm and controlled, this belongs more to elevated or written language. In everyday speech, you would more likely use words like 'ruhig' (calm) or 'gefasst' (composed/collected).
Der Zeuge machte vor Gericht einen sehr zusammengesetzten Eindruck.
The witness made a very composed impression in court.
simple / just
'einfach' and 'zusammengesetzt' can be opposites when talking about the structure of things or words. 'einfach' then describes something that is not complicated or seems simply built. 'zusammengesetzt' means that something consists of several parts. Learners may confuse the words because a compound word or system does not always have to be difficult. Still, the difference lies in the structure: 'einfach' emphasizes little complexity, while 'zusammengesetzt' emphasizes multiple components.

Im Kurs lernen wir, dass „Haustür“ ein zusammengesetztes Wort ist.
In the course, we learn that 'Haustür' is a compound word.
Die Mitarbeiterin wählt eine einfache Lösung, weil wenig Zeit bleibt.
The employee chooses a simple solution because there is little time left.