Connections between the Principal and the Subordinate Clause.
Sub-clauses can be optional or indispensable for the structural and semantic integrity of the complex sentence. So, the sub-clause can be connected with the principal clause indispensably or optionally. Indispensable are the structures which occupy the position of the subject or the predicative ( Whoever comes will be welcome. Health is what he needs. What might be is not what is). These sub-clauses cannot be removed, otherwise it would make the structure ungrammatical. Optional are those clauses (attributive, adverbial) which merely give additional information to the antecedent (She speaks broken English, and she has a very foreign appearance which she exaggerates. A Christie). The sub-clause is joined to the principal clause syndetically, i.e. by a subordinating conjunction, or asyndetically, by adjoinment (with or without inversion) {Tumors of the brain often cause behaviors the layman might think as psychotic (St.King); Should he come, tell me about it}.
Neutralization between Subordination and Coordination.
The borderline between the compound and the complex sentence is very vague and the difference between them can be neutralized. It occurs when a sub-clause comes to be very loose, losing its grammatical tension. Neutralization between subordination and coordination is observed when 1. a temporal clause (a while-clause, or a when-clause) expresses contrast rather than time {His sense of property is extreme, while you have practically none (J.Galsworthy); He wears fine clothes while I go in rags (O.Wilde)}. 2. Neutralization occurs in structures with continuative attributive clauses {The casino had been dropping money lately which shouldn’t be (M. Puso. Godfather)}→ The casino had been dropping money lately and that shouldn’t be }.The conjunctive pronoun which is easily replaced here by the phrase and that.. 3. Neutralization occurs when a when-clause expresses a successive action {I was listening, and thinking how the wind assailed and tore it when I heard a footstep on the stairs .Ch. Dickens. Great Expectations)→ and then I heard a footstep on the stairs}.