Adverb clauses: show time, reason, condition, and contrast
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Use an adverb clause with a main clause to show time, reason, condition, or contrast. Start the adverb clause with a linking word such as when, because, if, unless, or although. If the adverb clause comes first, use a comma before the main clause. If it follows the main clause, a comma is not usually needed.
Because the train was delayed, we called the hotel.