Otherwise known as that, which and commas.
Are you ready? Strap yourself in for some scenarios.
SCENARIO IA: Chocolate that is not healthy should be banned.
SCENARIO IB: Chocolate which is not healthy should be banned.
SCENARIOS IA and IB (I prefer that but which is okay too) tell us that any chocolate that is not healthy should be banned: that/which is not healthy is a defining relative clause (DRC) containing essential information about the TYPE of chocolate that should be banned. In other words, some chocolate is healthy (probably the boring dark chocolate with high cocoa content) but much is unhealthy (sigh). The clue is in the punctuation (or lack of it).
SCENARIO II: Chocolate, which is not healthy, should be banned.
SCENARIO II is far more of a problem for chocolate addicts. It tells us that ALL chocolate is unhealthy (even the lovely vegan ‘milk’ and white chocolate that I occasionally binge on) and chocolate itself should be banned. The which is not healthy part is a non-defining relative clause (NDRC): the commas surrounding it tell us it is non-essential information. If you remove it, the sense remains: Chocolate should be banned. Note: That (in place of which) DOES NOT work in this scenario.
Relative pronouns are used to introduce relative clauses: who, whom, whose, which, that.
BUT you don’t always need a relative pronoun, for example when the relative clause is introduced by a zero relative (ZR). Say what?
A zero relative is the missing element at the beginning of a relative clause in which the relative pronoun has been omitted.
SCENARIO IIIA: It must have something to do with the chocolate that I am scoffing.
SCENARIO IIIB: It must have something to do with the chocolate I am scoffing.
SCENARIO IIIB rolls off the tongue more easily than IIIA: the relative clause is introduced by a ZR. You decide whether or not you want the relative pronoun, that, because it is the object of the relative clause (I (the subject) am scoffing the chocolate (the object)).
SCENARIO IV: She stopped in front of the fridge, which was full of cold chocolate.
In SCENARIO IV you cannot omit the relative pronoun, which, because it refers to the fridge, the subject of the relative clause. In standard English the ZR can only function as the object of the relative clause (as in SCENARIO III, the chocolate).
Complicated? We're not done yet…
SCENARIO V: When you buy chocolate that you really love and that is also healthy, you need to eat it all before someone else does.
In SCENARIO V there are two relative clauses (that you really love; that is also healthy). Although you could leave out the first that, omitting the second that would sound really odd, so it is better to keep them both in. For balance.
You could even turn it into a defining relative clause and a non-defining relative clause, adding commas: When you buy chocolate that you really love, which is also healthy, you need to eat it all before someone else does. You can remove the NDRC, which is also healthy, leaving the DRC, that you really love, and it still makes perfect sense.
I know. It’s a lot.
If you’ve made it through this, you deserve a square (or 10) of your favourite chocolate.