Some dependent adverbial clauses are incomplete and leave out 'both' subject and main verb. The subject of the main clause then becomes the subject of the dependent clause as well, with some meaning of the verb 'to be' understood (e.g. When buying a computer, you should seek professional advice = When you are buying a computer, you should seek professional advice). An abridged dependent clause is not possible if the subject of the main clause cannot logically function as the subject of the dependent clause (e.g. Though twenty-one, John's work is that of an eight year old. 'John's work' is the subject of the main clause and cannot logically be the subject of the abridged clause).