This document discusses reducing adverb clauses of time or contrast. It explains that when the subject of the adverb clause and main clause are the same, the adverb clause can be reduced by dropping the subject. It also notes that when the subjects differ, the clause cannot be reduced. The document also specifies that the way an adverb clause is reduced depends on the verb - if the verb is a form of "be", the subject and verb are dropped, and if another verb, the subject and helping verb are dropped and the -ing form is used.