ausstehend
outstanding
Not yet paid, completed, or delivered; due.
Es gibt noch einige ausstehende Rechnungen aus dem letzten Monat.
There are still some outstanding invoices from last month.

outstanding / pending
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outstanding
Not yet paid, completed, or delivered; due.
Es gibt noch einige ausstehende Rechnungen aus dem letzten Monat.
There are still some outstanding invoices from last month.

'Ausstehend' (outstanding/pending) is the present participle of the separable verb 'ausstehen'. It describes a state in which something is still missing, unfinished, or expected.
Die ausstehende Lieferung sollte morgen ankommen.
The outstanding delivery should arrive tomorrow.
The word is very often used as a predicative adjective with the verb 'sein' (to be) or 'bleiben' (to remain), often together with 'noch' (still). The structure is then: subject + sein/bleiben + (noch) + ausstehend.
Die Bestätigung für den Termin ist noch ausstehend.
The confirmation for the appointment is still pending.
In the meaning 'not yet paid or delivered', 'ausstehend' (outstanding) is typically combined with financial or logistical terms. Common collocations are 'ausstehende Zahlungen' (outstanding payments), 'ausstehende Rechnungen' (outstanding invoices), or 'ein ausstehender Betrag' (an outstanding amount).
Bitte überweisen Sie die ausstehende Summe bis Ende der Woche.
Please transfer the outstanding amount by the end of the week.
When a decision or information is expected, 'ausstehend' (pending) is often combined with words such as 'Antwort' (response), 'Entscheidung' (decision), 'Urteil' (judgment), or 'Ergebnis' (result).
Das endgültige Ergebnis der Untersuchung ist noch ausstehend.
The final result of the investigation is still pending.
open
'offen' and 'ausstehend' can both mean that something has not yet been dealt with, paid, or decided. 'offen' is more general and often sounds more everyday: a question, a problem, or an invoice is still open. 'ausstehend' emphasizes more strongly that you are still waiting for something, for example a payment, a delivery, a response, or a result. Learners may confuse the words because both describe an unfinished state. As a simple guide: 'offen' describes the state, while 'ausstehend' is especially appropriate when something still has to arrive or be dealt with.

Bei der Arbeit wartet er noch auf die ausstehende Antwort der Kollegin.
At work, he is still waiting for his colleague's pending response.
Nach dem Gespräch ist noch eine wichtige Frage offen.
After the conversation, one important question is still open.