unbezahlt
unpaid
Describes an activity, work, or internship for which you receive no money or salary.
Er leistet viel unbezahlte Arbeit für den gemeinnützigen Verein.
He does a lot of unpaid work for the nonprofit organization.

unpaid
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unpaid
Describes an activity, work, or internship for which you receive no money or salary.
Er leistet viel unbezahlte Arbeit für den gemeinnützigen Verein.
He does a lot of unpaid work for the nonprofit organization.

The word is made up of the prefix 'un-' and the participle 'bezahlt' (paid). The prefix 'un-' expresses negation, so 'unbezahlt' literally means 'not paid'.
Die Rechnung vom letzten Monat ist immer noch unbezahlt.
Last month's bill is still unpaid.
The context makes clear which meaning is intended. If people talk about 'Arbeit' (work), 'Praktikum' (internship), or 'Überstunden' (overtime), it refers to an activity without pay. If people talk about 'Rechnungen' (bills) or 'Schulden' (debts), it refers to an outstanding financial claim.
Sie macht ein unbezahlschtes Praktikum in einer Agentur.
She is doing an unpaid internship at an agency.
In connection with work, 'unbezahlt' (unpaid) is often used for activities that you do additionally or without compensation. Common combinations are 'unbezahlte Überstunden' (unpaid overtime), 'ein unbezahltes Praktikum' (an unpaid internship), or 'unbezahlter Urlaub' (unpaid leave).
Viele junge Leute müssen am Anfang ihrer Karriere unbezahlte Praktika machen.
Many young people have to do unpaid internships at the start of their careers.
In the meaning 'not settled', 'unbezahlt' (unpaid) is often used with the verb 'bleiben' (to remain). This expresses that a bill or debt remains outstanding over a longer period of time.
Trotz mehrerer Mahnungen blieben die Rechnungen unbezahlt.
Despite several reminders, the bills remained unpaid.
open
'offen' and 'unbezahlt' can both mean, in financial contexts, that an invoice or debt has not yet been paid. 'offen' is often used in offices, accounting, or everyday life when an amount is still outstanding. 'unbezahlt' says more directly that something has not been paid. Important: 'unbezahlt' can also describe work or an internship for which you receive no money. In this meaning, 'offen' does not fit. So the risk of confusion arises especially with invoices: an 'offene Rechnung' (outstanding invoice) is usually an 'unbezahlte Rechnung' (unpaid invoice).

Das Praktikum war unbezahlt, aber Lena hat viel gelernt.
The internship was unpaid, but Lena learned a lot.
Bei der Werkstatt ist noch eine Rechnung von letzter Woche offen.
There is still an invoice from last week outstanding at the repair shop.