Confusion Between Which vs That [duplicate]

I am having a doubt between the usage of which and that.

Which one of the following is correct and why?

The cougar is a member of the cat family that grows around 8 feet in length.

or

The cougar is a member of the cat family which grows around 8 feet in length.

Hope you can clarify it ASAP.

Solution 1:

To understand the use of "that" & "which" like the way you sentenced, you need to know about restrictive/nonrestrictive modifiers.

For example, the following two sentences are both correct. But conveys totally different meaning.

The cars that are expensive often get stolen.

The cars, which are expensive, often get stolen.

A restrictive modifier restricts the scope of the noun to a subset. This is where "that" comes into play. In the first statement, "The cars that are expensive often get stolen" it means only those cars that are expensive get stolen. Not all cars!
We are pointing out a subset of cars called "expensive cars".

A non-restrictive modifier adds more information to the noun. Even if you remove the modifier the core meaning stays the same. In our example, "The cars, which are expensive, often get stolen" it means all cars are expensive and they get stolen. And it also means all cars get stolen.

Both of the below statement means the same. But the first merely adds more information.

The cars, which are expensive, often get stolen.
The cars often get stolen.

Also a non-restrictive modifier needs to be set off from the noun it modifies by commas. Whereas restrictive modifier should not have any commas. This is also a prime differentiating factor.

The cars that are expensive often get stolen.

The cars, which are expensive, often get stolen.

Solution 2:

I have two tests. [1] Bear in mind that 'that' defines, while 'which' describes. To remember this you can mutter to yourself the phrase: "This is the house that Jack built."

[2] The comma test; if you would be inclined to put commas round the clause then 'which' is likely to be more appropriate.

"The cougar is a member of the cat family that grows around 8 feet in length." Does not need commas, so 'that' can stand. The phrase suggest that at 5 foot the cat cannot be a cougar.

The cougar, which frequently grows to around 8 feet in length, is a member of the cat family. This can take commas and 'which'. (Unfortunate the borderline is indistinct.)