A complex sentence is a polypredicative unit built up on the principle of subordination. which varies from a close to a very loose connection with many gradations in between. The constituents of the complex sentence are traditionally called clauses. Transformationalists advanced the term “included structures” as the syntactically dependent part of the complex sentence is included (embedded) into the independent (non-included) structure.
The complex sentence of minimal composition includes two clauses – a principal clause and a subordinate clause. The two are interconnected, the very existence of either of them is supported by the existence of the other. Sub-clauses can adjoin the main clause or they can be embedded (included) into the main clause (We have engineered toys and gadgets wedon’t understand and technological terrors we may not be able to control (St. King).
The Status of the Subordinate Clause.
The most accepted view is that a sub-clause is an explicitly predicative structure with a subject and a predicate. Many scholars hold that a clause needn’t have explicit predication (H. Poutsma, G.Curme, M.Bryant, R. Long, etc.). They treat participial, gerundial and infinitival complexes as undeveloped verbid sub-clauses (Timothy had left a lot of money, with nobody in particular to leave it to (J. Galsworthy). Prof. Y.G.Birenbaum treated participial, nounal, adjectival phrases introduced by the conjunctions while, unless, when, until as elliptical sub-clauses (He will continue talking until stopped. When traveling I have now and then watched the sunrise). We can transform these sentences into complex sentences with sub-clauses ( Until they stopped him, he will continue talking. When I was traveling, I now and then watched the sunrise). Still most linguists find these formations merely phrases, carrying implicit predicative relations.