Significa: "Only if you don't come, I will go to the party ( without you)" Respuesta: «A no ser que vengas, voy a ir a la fiesta». «A no ser que no vengas, voy a ir la fiesta». Espero que te sirva Carol. Saludos cordiales. Chris CG.

Your sentences are not actually correct, and they are conditional sentences, not sentences with double negatives. A double negative is when you use 2 negative elements (words) in a sentence, and it is important to know that in many cases using a double negative is considered incorrect or poor English because these negative elements cancel each other out. Here are some examples of double negative sentences: she never danced with no one. I don't know nothing. These are both double negatives and are incorrect English. They should be; she didn't dance with anyone and I don't know anything. One of the most famous uses of double negatives can be found in the Pink Floyd song 'Another Brick in the Wall' where they sing about how 'we don't need no education'.

Carol, let me try to explain: Unless = a no ser que / a menos que / salvo / excepto. Then if you translate it to English ("a no ser que no vengas") should be: "unless you don't come". In Spanish it could be also written so: "Salvo que tu no vengas, iré a la fiesta" or "A menos que tu no vengas, iré a la fiesta". Best regards