“Subordination: In grammatical theory, a relationship between two units in which one is a constituent of the other or dependent on it. The subordinate unit is commonly a subordinate clause organized ‘under’ a superordinate clause. Such organization can be described in two ways: the subordinate unit as a constituent of the superordinate unit and the subordinate unit as dependent on but distinct from the superordinate unit. In the sentence, They did it when they got home, the subordinate when-clause may be either a constituent of its superordinate main clause, which begins with They and is coextensive with the entire sentence, or dependent on a more limited main clause They did it. There is in principle no limit (apart from comprehensibility and practicality) to the subordination of clauses one under another. In the sentence, They saw that I was wondering who won the competition, the subordinate who-clause is a constituent of or dependent on its superordinate that-clause (which ends with the competition), while the that-clause is also a subordinate clause, in turn a constituent of or dependent on its superordinate clause beginning with They. Subordinate clauses may also be constituents of or dependent on phrases: in What’s the name of the woman who’s winning the competition?, the who-clause modifies the noun woman.
Form. Traditionally, part of a sentence can only be classed as a subordinate clause if it contains either an identifiable or an ‘understood’ finite verb. In contemporary grammatical analysis, however, subordinate clauses may be classed as: finite (‘I think that nobody is in’); nonfinite (‘He used to be shy, staying on the fringes at parties’); verbless (‘She will help you, if at all possible’), Traditionally, the second category would be classed as a participial phrase and the third as a clause with the verb ‘understood’ (it is). Finite subordinate clauses are usually marked as subordinate either by an initial subordinating conjunction (after in He got angry after I started to beat him at table-tennis) or by an initial wh-word that also functions within the clause (who in Most Iranians are Indo-Europeans who speak Persian, where who is the subject of the subordinate clause). These subordination markers sometimes introduce nonfinite clauses (while in I listened to the music while revising my report), and verbless clauses (if in If necessary, I’ll phone you).
Function. Subordinate clauses fall into four functional classes: nominal, relative, adverbial, comparative. Nominal or noun classes function to a large extent like noun phrases: they can be subject of the sentence (‘That he was losing his hearing did not worry him unduly’) or direct object (‘He knew that he was losing his hearing’). Relative or adjective/adjectival clauses modify nouns: the that-clause modifies star in ‘She saw a star that she had not seen before.’ Adverbial of adverb clauses function to a large extent like adverbs: the adverb there could replace the where-clause in ‘You should put it back where you found it.’ Comparative clauses are used in comparison and are commonly introduced by then or as: ‘The weather is better than it was yesterday’; ‘The weather is just as nice as it was yesterday.
All such clauses occur in complex sentences. Subordination contrasts with coordination, in which the units, commonly the clauses of a compound sentence, have equal status: the clauses joined by but in We wanted to see the cathedral first, but the children wanted to see the castle straight away. Sentences in which both subordinate and coordinate clauses occur are compound-complex sentences: with before and but in We wanted to visit the cathedral before we did anything else, but the children wanted to see the castle straight away.”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.