"Unless the door is open, the machine can start" is logically correct, but seems kind of "back to front" and therefore an unlikely thing to want to say. This is because "Unless X, Y" normally implies that we don't want Y to happen. Here, we do (presumably) want the machine to start.
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ROBIN UKCan we say ( Unless the door is open , the machine can star ) , and if its not why?Use quotes or italics, not parentheses, for the example sentence you're referring to.