Collins Good Grammar. Graham King

which is the subordinate clause because it can’t stand on its own:

main clause	subordinate clause
This is the patient	who stopped breathing.

      You can add more information to a subordinate clause, but regardless of how much extra information you pile on it remains a subordinate clause because it is always subordinate to the main clause, serving only to influence the word patient:

main clause	subordinate clause
This is the patient	who, when I visited the hospital yesterday, stopped breathing for several minutes.

      It’s worth spending a little more time with subordinate clauses because no matter how long they are they have the ability to function as nouns, adjectives and adverbs. Understanding this should help you construct more precise and efficient sentences.

      We recognise three kinds of subordinate clauses:

NOUN CLAUSE – where the clause acts as a noun.	She told him what she thought.What’s right and what’s wrong are the questions at the heart of true civilisation.I told him that Judy was coming.
ADJECTIVAL CLAUSE – where the clause acts as an adjective.	This is the door that won’t close properly.The parcel that’s just arrived is for you.London is the place which offers the greatest opportunities.
ADVERBIAL CLAUSE – where the clause acts as an adverb.	You should go there before the shops open.Because it began to rain I had to buy an umbrella.We were quite upset when John came in.

      Look at the noun clauses closely and you will see that they really do function as nouns. Try mentally rewriting the sentence She told him what she thought as She told him [thoughts] and – Presto! –the clause becomes a noun.

      Do the same for the adjectival clauses. What the sentence This is the door that won’t close properly is saying is, This is the [jammed] door. The adjectival clause that won’t close properly is merely substituting for the adjective jammed. Incidentally, you may have noticed that the adjectival clauses are introduced by the relative pronouns that and which. These and who, where, whose, what and as are typically used for this purpose, which is why you may sometimes see adjectival clauses referred to as relative clauses.