So I'm a new DM and one of my PC's who's a 1st-level warlock wants to multi-class into cleric. So he would essentially be a warlock 1/cleric 1.
Since the Warlock only has one 1st-level spell slot at 1st level while clerics have two at 1st level, do they stack, or do you just have two 1st-level spell slots? How exactly does this work?
Also, how would the number of cantrips change?
In Chapter 6 of the PHB, under "Spellcasting" (p. 164), the following paragraph can be found (with additions by me in brackets):
Pact Magic. If you have both the Spellcasting class feature [e.g. via cleric] and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots from the Spellcasting class feature to cast warlock spells you know.
Since the Spellcasting and Pact Magic class features are separate class features, although they both give you spell slots, they are still different class features, so the spell slots "stack" because they come from different sources.
Of course, when you short rest, you only get the slots back from your Pact Magic, you must complete a long rest to get all of them back (i.e. those from a cleric's Spellcasting).
Yes, cantrips stack too. You get 2 cantrips via warlock and 3 cantrips via cleric; these cantrips must come from their respective spell lists, but otherwise, having cantrips from one source does not exclude you from gaining more cantrips from another source (so long as you stick to the spell list of the class granting you the cantrips at that time).
You use the multiclass spellcasting rules from the Player's Handbook (also in the basic rules in DnDBeyond)
Warlock spellcasting is a bit different and use the "Pact magic" feature. This is specifically mentioned in the bit about multiclassing and spellcasting. Essentially you gain the spell slots from your warlock levels and the spell slots from any other spellcasting classes (like cleric). So, yes, they stack.
(You may also use the spell slots from one class to cast the spells you know from the other class, as it says).
