How many of us have paused during a translation to scratch our heads and ask, “now should that be a ‘that’ or a ‘which’? and what about the comma?”
Well, it all depends on your perspective... and your locale...
In US English, “that” introduces what is called an essential, or restrictive, clause. Essential clauses add information that is vital to the point of the sentence. “Which” introduces a non-essential, or non-restrictive, clause. Non-essential clauses add supplementary information and are frequently offset by commas.
Interchanging the two structures is grammatically incorrect in American English because they have different meanings.
In the interest of making things as simple as possible, US English applies the following rule: Use “that” before a restrictive clause (information required for the sentence to make sense) and “which” before everything else.
Here's an example of a restrictive clause, using “that”:
Gems that sparkle often elicit forgiveness.
The words “that sparkle” restrict the kind of gems you're talking about. Without them, the meaning of the sentence would change. Without them, you'd be saying that all gems elicit forgiveness, not just the gems that sparkle.
Here's an example of a non-restrictive clause, using “which”:
Diamonds, which are sparkly gems, often elicit forgiveness.
Diamonds are gems by definition and always sparkle, so leaving out the words “which are sparkly gems” doesn't change the meaning of the sentence.
Here, things are both more straightforward and more confusing. UK English has mostly lost the distinction between “which” and “that” in restrictive clauses. Meaning, you can use “which” with both restrictive and non-restrictive clauses.
However, this often makes such sentences ambiguous, forcing the reader to infer the distinction from context.
- Gems which sparkle often elicit forgiveness
In ANY English dialect, “that” can ONLY introduce restrictive clauses. In other words, non-restrictive clauses MUST start with “which”. So, all English speakers do have a distinction between “which” and “that”, it’s just that UK English doesn’t have a strong distinction when it comes to restrictive clauses.
Therefore, when in doubt, your safest bet is to apply the stricter “‘that’ before restrictive and ‘which’ before everything else” rule, and use commas to offset those non-restrictive/non-essential clauses.