Conditional connector 'es sei denn' (unless)

es sei denn(, dass) means unless. It introduces the single exception that would cancel the statement just made: the main clause holds except in the case named after es sei denn.

It is a fossilised Konjunktiv I form - sei is the old subjunctive of sein - preserved as a fixed phrase:

• Ich gehe nicht, es sei denn, du kommst mit. - I won't go unless you come along.
• Der Termin bleibt bestehen, es sei denn, es tritt etwas Unvorhergesehenes ein. - The appointment stands unless something unforeseen happens.
• Wir veröffentlichen den Bericht, es sei denn, dass rechtliche Bedenken bestehen. - We will publish the report unless there are legal objections.

You can continue either with a main-clause word order (after the comma) or with dass + verb-final clause; both are correct.

Common mistakes:

• "Correcting" sei to ist - the Konjunktiv I sei is obligatory in this frozen idiom.
• Dropping the comma before es sei denn; it always begins a new, set-off clause.
• Using it where you mean außer + noun - es sei denn introduces a clause, not a noun phrase.

'es sei denn' (unless) introduces an exception.

Means 'unless'; introduces the one exception. Fixed form with 'sei'.

'es sei denn' nennt eine Ausnahmebedingung (außer wenn).