i am an american exchange student in austria so i have several opportunities to correct papers. sadly, i do not know the difference of "which" and "that".
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" He was the most eloquent speaker that I ever heard " , ( that is better than which ) because the superlative adjective ' the most ' used in the first clause.

After the words, all, same, any, none, nothing, only, as, ( use that )Emotion: smile
These two words can be confusing. Here's a good rule of thumb:

Use "that" whenever the identity "that" is replacing is known to the speaker. Examples:

I will read the book that is written by my favorite author. (not: "which is written..")
All that I can see from here is beautiful.
She didn't even apologize, and that is why I am angry. (not: "apologize, which is why...")

Use "which" whenever the speaker does not know what "which" is. Examples:

Which route will we take to get there?
I do not know which [one] is the better car.
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The man who was crossing the street was stuck by a car that Mr.smith was driving
Some people adhere to a rule that says in effect use "that" for restrictive relative clauses and "which" for non-restrictive relative clauses. A good discussion of this topic can be found [url="http://www.bartleby.com/64/C001/062.html "]here[/url].

My own personal belief is that this rule has been relaxed over time. Now you can use "that" and "which" much more interchangably with restrictive and non-restrictive relative clauses. But some still rigourously adhere to this rule.

I hope that helps.
The relative pronoun "which can introduce both restrictive and non restrictive relative clauses, as you will read in any fairly good grammar.

"The house which I bought last year has three bedrooms." (restrictive)
"The house, which I bought last year, has three bedrooms." (non-restrictive)

On the other hand, you will probably not find "that" as the introductory pronoun of a non-restrictive relative clause in a text written by anyone who has some knowledge of grammar.

"The house (that) I bought last year has three bedrooms."
"The house, that I bought last year, has three bedrooms." incorrect

"The toys that are in the garden are not my children's."
"The toys, that are in the garden, are not my children's." incorrect

In the "incorrect" sentences, "which" is the right pronoun to use.

There is yet another type of relative clause in which "which" is preferred to "that": what is called "sentential relative clause" by some authors because it does not modify any preceding nouns in particular but a whole clause:

"John was late for dinner last night, which made me angry."

In this sentence, "which" refers to/replaces the whole clause "John was late for dinner last night". Some teachers tell their students that a good way to recognise this use of "which" is to see if it can be replaced by "the fact":
"John was late for dinner last night; 'the fact' made me angry."

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I know which /that is the better car. .....in this sentece which has to be used - which or that? as speaker knows what hes talking about
This thread confuses several different uses of the two words in its discussions. In your case:

I know which is the better car-- the speaker knows but has not revealed the information.
I know that is the better car -- this 'that' is not a relative pronoun; it is a demonstrative pointing to the better car, so the information has been revealed. The same sentence can also be cast as: I know (that) that is the better car.
but don t we know which here.isn t it obvious we are talking about the car...so which is known by the speaker.
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