It's awkward to begin a sentence with "unless" in English. "Except" doesn't work either. You should use "If" to express a conditional. "If you don't come, I will go to the party". Tengo pregunta para ti. ¿Puedo escribir "Si no vengas, yo iré a la fiesta"?

That's really awkward. I assume that you mean that if your friend goes to the party, then you will go. But if your friend doesn't go to the party, then you won't go. Do I understand you correctly? If you must use "unless" then - "I'll go to the party unless you don't go." or "Unless you go to the party, I won't go." But I'd prefer to use an "if...not" construction.

Hola Mauricio, your first statement is all right, don't use except it would sound weird. The only aspect I found wrong on your question is when you asked the question, you wrote ''I want express,'' the correct way is ''I want to express''. espero te ayude en algo, also you may use ''if'' it would work as good as unless.