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> Unless disciplined, a dog becomes a household pest.
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Is the use of unless in this sentence correct? It sounds to my naive ear to be fine, but I always think of unless as a conjunction, which does not seem to be the case here. I looked up *unless* in an online dictionary and discovered that it can additionally be used as a preposition in the sense of "except for", but again that would not seem to apply to this sentence as disciplined is an adjective. Since the sentence is taken from *Strunk and White* and sounds right to my ear, I am sure it is correct, so my main interest is to discover what the underlying construction is here.