What is the difference between defining&non-defing clauses?

You can tell the difference between defining and non-defining clause from the use of comma.

The function of a defining relative clause is to limit the application of the antecedent; where that is already precise, a defining clause is not wanted. The limitation can be effected in more than one way, according to the nature of the antecedent. As a rule, the antecedent gives us a class to select from, the defining clause enables us to make the selection. Thus in our first example the antecedent leaves us to select from the general class of 'men', the defining clause fixes the particular man (presumably the only man, or the only man that would occur in the connexion) 'who called yesterday'. Sometimes, however, the functions of the two are reversed. When we have an antecedent with a superlative, or other word of exclusive or comprehensive meaning, such as 'all', 'only', 'any', we know already how to make our selection, and only wait for the relative clause to tell us from what class to make it. We know that we are to choose 'the best novel': the relative clause limits us to the works of Trollope. We are to choose 'any man' we like, provided (says our relative clause) that he 'knows three words of Greek'. In either case, the work of definition is done by the exclusion (implied in the relative clause) of persons or things that the antecedent by itself might be taken to include.

Example:
Defining Clause
Andy who dines with us tonight is not the Andy who was at school with you.


Non-defining Clause
Henry, who should know something of the matter, thinks differently.

2007-08-01 14:51:58 補充：
When the sentence is important to the subject, you use defining clause (no comma).When it is not vital to the subject (just for additional information), you use non-defining clause (with comma in front and at the end of the sentence with additional information).

評唔到正面sorry

if the sentance is Extra that mean whether u can have it or not -- non-defining
Ex. Jane, who is 25-year-old and very tall, is a teacher.

Otherwise, if the sentance is important, u must write down -- defining
Ex. The girl in the library, who has long hair is my teacher.