Pardon me. I think your real actual question is when to use which / that?
I will give you a hint - its not "iron clad". These are from my notes and almost near to perfection on the real exam. Just kidding

1. (edit)
which always goes with comma. If you don't see comma before which - eliminate

Exception to this rule :
It's true we need a comma before "which" but in case "which" is preceded by a preposition such as "in which", ''of which'', ''from which'', etc. We don't need a comma in such cases before "which".

e.g. The company of which I'm a member is going to bankrupt soon.

2. which is used for the optional information (followed by comma) - so even if you cross / strike the information - the meaning will not change.

Whereas "that" is used for essential information. The information attached to "that" cannot be removed from the sentence without changing the meaning of the sentence.

3. (edit)
that can refer to non living, animal
which can refer to non living, animal
Use personal pronoun "who" to refer to human

4. GMAT doesn't like pronoun ambiguity - so if you CANNOT resolve the references of "that" or "which" - eliminate that choice confidently.

5. X of Y, which -> This pattern is rarely tested. "which" refers to the noun which complies with SV agreement.

the box of nails, which is on the counter, is to be used on this project.

In the above sentence which refers to "box". Note the singular "is". For Ron's explanation refer -
http://www.manhattangmat.com/forums/pos ... tml#p31162

PS : I have incorporated the suggestions from the post.

i strongly suggest you go thru og problems where 'which' is used correctly and incorrectly, there are a lot btw, then search thru each problem on either the Manhattan gmat or Gmatclub forums, there you will find your answer!

1. which always goes with comma. If you dont see comma before which - eliminate

I agree with you on the above statement but there is one exception to this rule which we all need to remember always. It's true we need a comma before "which" but in case "which" is preceded by a preposition such as "in which", ''of which'', ''from which'', etc. We don't need a comma in such cases before "which".

e.g. The company of which I'm a member is going to bankrupt soon.
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I doubt whether this statement is correct. Since the GMAT doesn't allow "that" to refer to "human" (According to MGMAT SC Guide). Correct me if i am wrong.

The other things worth mentioning are -
1. "WHICH" with comma - use "which" only to refer to the noun immediately preceding it. Never use "which" to refer to the entire clause.
2. "WHICH" must modify things.
3. That Rule - Accordning to GMAT "THAT" cannot be used to modify people.

The last time I checked...who, whom, and whose can refer to human. Correct me if I've missed something.

Whose - can refer to both human and things..

another out of the box, rarely tested use of WHICH:

I quote words from an MGMAT expert here:

the gmat tends to write sentences in which "which" stands for the ELIGIBLE noun that's closest to the comma. by "eligible", i mean that the noun has to AGREE IN TERMS OF SINGULAR/PLURAL with the FOLLOWING VERB. here's an example: the box of nails, which is on the counter, is to be used on this project. in this case, "which" CANNOT refer to "nails", since the verb "is" is singular. therefore, the nearest eligible noun is "box (of nails)". so, "which" unambiguously stands for that. in our observation, the gmat has been VERY good about this. whenever i've seen a "which" that refers to "X of Y" rather than just Y, it has ALWAYS been the case that X was singular and Y was plural (or X was plural and Y was singular), and the verb had a form that matched X and didn't match Y.

Also that cannot be used for humans.

brilliant addition rohu27 !
Indeed, the GMAT tries to trick test-takers with the rule...

Good discussion topic here.

In general - "which" is used to DESCRIBE a phrase.

"That" is used to restrict the scope of what you are talking about.

For example,

"I'm going to the park that has tennis courts."
instead of
"I'm going to the park, which has tennis courts."

You can assume there are lots of different parks. Which park are you going to? You're going to the one THAT has tennis courts.
When you are restricting the scope of ALL PARKS down to a specific one--then you use THAT.

Here's another example:
"I'm going to Mountain Lake Park, which has tennis courts."
instead of
"I'm going to Mountain Lake Park that has tennis courts."

Here, "has tennis courts" describes a specific park Mountain Lake Park. It does not restrict the scope of parks because the scope is already restricted by giving a name to the specific park we are talking about. Since the phrase "which has tennis courts" is a DESCRIPTION that is NOT RESTRICTING the scope of our perspective, then we use "which" instead of "that."

For more examples, watch these two videos here.

I think 1 thing is missing ...
Which should never be used to modify a clause , use it only to modify a noun.
Instead use a verb+ing participle to modify the clause.
Example:

The police control got very strict , which led to some amount of discipline in the city.

The police control got very strict , leading to some amount of discipline in the city.

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