In Spanish you can indicate with a clause introduced by salvo que that there is an exception to something mentioned in the previous clause. There are two ways of using and interpreting salvo que.
Salvo que + Modo indicativo
We use salvo que followed by the indicative to talk about an exception that is real. For example:
Todo fue sobre ruedas, salvo que la calidad del sonido no era muy buena.It all went smoothly, except that the sound quality wasn't very good.
All these examples show an exception, some kind of limitation to what it said in the first clause. The meaning here is very similar to the English usage of but or although.
Tu redacción es tan buena como la mía, salvo que/pero/aunque yo he usado un lenguaje más formal. (except that, but, although)
Here one can also use the more literal "excepto que...":
Tu redacción es tan buena como la mía, excepto que yo he usado un lenguaje más formal. (except that)
Salvo que + Modo subjuntivo
We use salvo que followed by the subjunctive to talk about an exception presented as a possibility, not as a reality. In this case, we can alternate between salvo que, a menos que or a no ser que, and the meaning is usually translated as "unless". For example:
No voy a firmar ese contrato, salvo que incluyas una cláusula que indique claramente mi horario.I'm not going to sign that contract, unless you include a clause indicating clearly my working hours.
Pues, terminamos aquí la sesión de hoy, a no ser que alguien tenga alguna pregunta.Well, we'll finish today's session here, unless someone has a question.
No van a aceptar su propuesta, a menos que les convenza con algún otro incentivo.They are not going to accept your proposal, unless you convince them with some other incentive.
In this case, used with the subjunctive, the clause with salvo que, a menos que, a no ser que is seen as a "conditional clause", a condition to be fulfilled in order to make something happen.
Remember that salvo que, a menos que and a no ser que are interchangeable, with no change in meaning.
When used with the subjunctive to mean "unless" one could also use the conjunction "excepto", although this is more frequently followed by "si": excepto si + subjunctive, for example:
Nunca te diría que sí, excepto si se lo pidieras de rodilla.
Note that with the first meaning expressed in 1. using the indicative, you cannot use a menos que and a no ser que.
See also Using "menos" to say except in Spanish to learn about a more basic use of "excepto".
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Examples and resources
Tu redacción es tan buena como la mía, salvo que yo he usado un lenguaje más formal. Your essay is as good as mine, except that I used more formal language.
Pues, terminamos aquí la sesión de hoy, a no ser que alguien tenga alguna pregunta.Well, we'll finish today's session here, unless someone has a question.
No llegarás a tiempo a la estación, a menos que salgas ahora mismo.You won't get to the station on time, unless you leave right now.
Nunca te diría que sí, salvo que se lo pidieras de rodillas.She'd never say yes, unless you asked her on your knees.
Me encantan los dulces de esta pastelería, salvo que la dueña es un poco antipática.I love the cakes from this bakery, except that the owner is a bit rude.
No voy a firmar ese contrato, salvo que incluyas una cláusula que indique claramente mi horario.I'm not going to sign that contract, unless you include a clause indicating clearly my working hours.
