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}.