Words can be confusing sometimes and grammar books instead of resolving the confusion traps us more. Like the use of ‘that’ and ‘which’ can be such a task! It’s so daunting that people avoid using them and hang on to petty punctuation marks for support. Anyhow, here’s another brave attempt to put an end to this ‘albatross’ of a confusion because one must never abandon hope!

Consider this; You saw an amazing crystal vase at a friend’s place and can’t wait to talk about it with your wife. Hoping that she isn’t a grammar high-brow, you can escape the ire by using either of the following:

“The beautiful crystal vase that we saw at Frank’s place”
OR

“The beautiful crystal vase which we saw at Frank’s place”

And if your wife is a grammar snob then why bring up the vase anyway?

The choice between ‘that’ and ‘which’ depends on something called restrictive and non-restricitve clauses. Clauses are part of a sentence with a subject and verb but are not complete on their own.

‘That’ is used in sentences with restrictive clauses that is they’re essential or restrictive. Example:
Buy shoes that are red in color.

Let’s go to that flower shop.

We are taking kids out for that toy movie.

‘Which’ is used in sentences that have non-restrictive and non-essential clauses. Example:

He bought patent shoes, which are red in color.

Let’s go to the flower shop, which is down the lane.

The toy movie, which released a week back is really good.

Also, no punctuation mark is used in sentences with restrictive clauses. In non-restrictive clauses a comma is used. For instance:

The building, which was made of steel collapsed.

Now you can fearlessly work up any conversation with your wife, including the one with ‘that’ or ‘which’.
And to be a complete ace at it, try and use both the words in the same sentence.

After all, the sentence that is easier to phrase is the one which is easier to understand.