A Comparative Study of “as” and “Which” in Non-Restrictive Relative Clauses

Both “as” and “which” can introduce a non-restrictive relative clause. But each of them has its own par- ticular function in meaning, part of speech and sentence structure. Therefore sometimes they can have similar usage in a certain situation, while most often they are quite different and can not be used alterna- tively.


Introduction
In any grammar book, the usage of "as" and "which" is illustrated, but the students are still confused in many cases and can not use them correctly in context. The usage of these two relative words is far more complicated and needs a detailed study of the different special and distinctive features.

Part of Speech and Meaning
In a non-restrictive relative clause, the use of "which" is not so complicated. It is used as a relative pronoun, the meaning of which is determined by the antecedent it refers to, and it can be interpreted as "that", "those things", "that case", and so on.
"As" is much complicated both in the part of speech and in meaning. Traditionally, just like "which", it is usually defined as a relative pronoun, as Professor Bo (2005: p. 536) and Professor Wang (2005: p. 491) put it. Some English dictionary also put it into pronoun category.
Defining "as" as a pronoun can make people understand easily which word it refer to, but can not make clear the meaning of it in context. Actually, "as" can not be understood so simply as a replacement for a word or words in the main clause, because it can has a special meaning in a certain situation .
So the 4th edition of Longman dictionary (2004) regards "as" as a conjunction: Example 1. David, as you know, has not been well lately (as: used to say that what you are saying is already known or has been stated before).
And in the Oxford English-Chinese dictionary (1997) there is no pronoun to define "as", with the following examples used as conjunctions: Example 2. Cyprus, as you know, is an island in the Medi-terranean (as: a fact which). Example 3. She's unusually tall, as are both her parents (as: and so too).
The 6th edition Oxford dictionary (2004) interprets "as" as: "used to make a comment or to add information about what you have just said" with the following two examples: Example 4a. She's very tall, as is her mother. Example 4b. As you know, Julia is leaving soon. Therefore "as", when introducing a non-restrictive relative clause, serves syntactic function as a special relative pronoun with the function of a conjunction. It can be used to refer to something in the main clause in some case, and meanwhile to add some special information to the main clause, such as "manner", "reason", and so on, according to the context.

Syntax and Structure Function
Both "as" and "which" can be used in a non-restrictive relative clause to refer to a part or the whole main clause. In some situation, they can be used alternatively. But sometimes can not. The following principles should be considered.

Principle of Contextual Meaning
The meaning of the contextual clauses is the key point in choosing "as" or "which" in the relative clause. Since "as" has its own special meaning, the clause it introduced is a supplement to the main clause and the meaning of the main clause and the non-restrictive relative clause should not be against each other. Quirk (1985: p. 1117) regards this type of clause as a comment clause which needs a requirement that "the as-clause must be semantically congruent with its matrix clause" (the following examples with * is not correct):   *as was unexpected. which was unexpected. 5. She has married again, *as was disgraceful.
which was disgraceful. Quirk, 1989 p. 1117 Zhang (1997) puts it further that the as-clause must be af-firmative while which-clause can be a negative:

 
which I can't bear 6. Mummy treats me just like a baby, *as I can't bear. p. 1406 Example      So according to such a principle of meaning, the "as" or "which" in the following examples can not be replaced by another.
Example 8. He never keeps his promise, which we don't like at all (Zhang & Sun, 2006: p. 340).

Principle of Grammatical Structure
1) If the relative words are used as a predicative or an object in the relative clause, both "as" and "which" can be used to introduce a non-restrictive relative clause.
Example 10. He wants to be useful, which (or as) he has never been (The antecedent here is "useful") (Zhang & Wen, 1998: p. 359).
Example 11. He is fond of music, which/as I'm glad to hear (Zhang, 1997: p. 1406) (The relative word here refers to the whole main clause). Example 12. He witnessed a boa, as he had never dreamed of in his childhood (Du, 2002: p. 601) (The antecedent of "as" is the whole clause "he witnessed a boa").
(But if "which" is used in the example 12, the meaning of the sentence is ambiguous. The sentence "He witnessed a boa, which he had never dreamed of in his childhood" can mean the same with example 13 that he had never had such a strange dream, but also has the meaning of "He witnessed a boa one day but he never had dreamed of a boa before").
2) When the relative words is used as an subject in the nonrestrictive relative clause, and the predicate in the relative clause is a non-transitive verb, "be" or other copula, both "as" and "which" can be used. Otherwise, "which" is a correct choice.
Example 13. The ships were frozen in, as/which not infrequently happens in those regions (Zhang, 1997(Zhang, : p. 1406

Principle of Position
As-clause is quite flexible about position, at the beginning, in the middle or in the end of a sentence, while the which-clause is usually in the end of a sentence. The followings are more examples bedside the previous ones. But when the antecedent of "which" is one word in the main clause, the position of "which" depends on the position of the antecedent. And when "which" refers to a clause, which-clause can be in the middle of a sentence functioned as a parenthesis.
Example 19. They were, which/as was a great honour to them, in Special Usage of "Which" "Which" ca an object and be co h fact is important (Zhang, 1997: p. Example 22. He may come, in which case I'll ask him vited to the wedding ceremony (Du, 2002: p. 579).
n be used after a preposition to be fore a noun to be a determiner, while "as" can not. Example 20. There are many books, some of which are Mallm's (Wang, 2005: p. 665