Learn English – What part of speech is “unless” in the following sentence

conjunctionsgrammaticalityprepositions

Unless disciplined, a dog becomes a household pest.

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.

Best Answer

In the sentence,

Unless disciplined, a dog becomes a household pest

unless is a conjunction*. This sentence is really a reduction of

Unless it is disciplined, a dog becomes a household pest.

The conjunctional function becomes clearer upon a rearrangement of the clauses:

A dog becomes a household pest, unless [it is] disciplined.

The most likely constructions a native speaker would use are the following:

  • A dog becomes a household pest, unless it is disciplined.
  • Unless it is disciplined, a dog becomes a household pest.
  • Unless a dog is disciplined, it becomes a household pest.
  • Unless disciplined, a dog becomes a household pest.

*I see how unless could be said to be used as a preposition:

A dog becomes a household pest unless disciplined.

(No comma before unless and disciplined becomes an adjective.) I doubt this is formal usage, though. Before the independent clause, however, I would always take unless as a conjunction.

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