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:
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.
Here one can also use the more literal "excepto que...":
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:
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:
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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