a no ser que
conjunctionCEFR B1
What does “a no ser que” mean in English?
unless
unless (a no ser que + subjunctive — always; slightly formal vs a menos que)
Example sentences
El vuelo sale a las seis de la mañana, a no ser que haya algún retraso por el mal tiempo en el aeropuerto de origen.
The flight leaves at six in the morning, unless there is some delay due to bad weather at the origin airport.
Pensábamos terminar antes del viernes, a no ser que surgieran complicaciones de última hora.
We thought we would finish before Friday, unless last-minute complications came up.
Asistiremos a todas las sesiones del congreso, a no ser que nos sea imposible por compromisos previos.
We will attend all the conference sessions, unless it proves impossible due to prior commitments.
How to use it
A no ser que means 'unless' and always triggers subjunctive. It introduces a single exception that would negate the main clause: 'Iremos, a no ser que llueva'. It is nearly synonymous with a menos que and the two can be used interchangeably in most contexts, though a no ser que is considered slightly more formal or literary. The subjunctive is invariable — a no ser que always takes it regardless of tense or polarity of the main clause.
Common mistake
A no ser que always takes subjunctive — it is in the 'always subjunctive' category. Do not use indicative: *'a no ser que llueve' is wrong. Note that the main clause is often positive (affirming what will happen) and the unless-clause qualifies it with a single exception. A no ser que and a menos que are interchangeable — use whichever sounds more natural.