I don't get the spell slot system for D&D 5e's Warlocks

Question

Now just recently I have had a group of friends get into D&D 5e and I ran into an issue with the Warlock class Spell Slots.

Cantrips: a spell that can be cast at will, without using a spell slot and without being prepared in advance. Repeated practice has fixed the spell in the caster’s mind and infused the caster with the magic needed to produce the effect over and over. Pretty obvious. You know as many as your class level tells you you can know.

Spells Known: How many spells your character can know excluding cantrips. My only question for this is if old known spells can be switched out for new higher level ones?

Spell Slots: the slot leveling I understand, but the whole basis behind it is what confuses me. Are spell slots their own separate choice of spell similar to how cantrips are separate from your spells known? Do you choose spells to know and out of the known spells you prepare ones to slots and they are cast at the slot level, not the normal level? When consumed for casting a spell, are they consumed on known spells and slot spells or just slot spells?

I just need some explanation. I'm a bit more of a visual learner so instead of someone just explaining what a spell slot is, please add a couple examples. (Ex. Warlock at 1st-9th level and a small scenario or two in which they would use a spell they know and a spell slot.) I want to know their spellcasting and how they progress level wise as much as I can.

Answer 1

Spell slots are the fuel you spend to cast spells.

For now, think of cantrips as being totally separate from spells.

All spellcasters, including warlocks, have a list of spells they can cast, through some combination of knowledge and preparation (the details vary by class). These spells all have levels — comprehend languages is a first-level spell, shatter is second-level, hold monster is fifth-level, etc.

Then (PHB, p.201):

When a character casts a spell, he or she expends a slot of that spell’s level or higher.

So to cast comprehend languages, you need to "spend" at least a first-level spell slot, but to cast hold monster you need to spend a fifth-level spell slot.

Most spellcasting classes — warlocks being the exception — have different numbers of slots of different levels. For example, a 9th-level wizard has four 1st-level slots, three each of second, third and fourth-level slots, and one fifth-level slot.

Warlocks are a bit different because they have a smaller number of slots, but all of the slots are of the same level. At first level, a warlock has one first-level slot, but at 9th level they have two fifth-level slots (but no slots of any lower levels).

When you cast a spell using a slot of a higher level than the spell — for example, casting shatter (a second-level spell) using a fifth-level slot — the spell may have increased effects. Shatter does extra damage when cast using a slot higher than second level. Other spells, such as comprehend languages have the same effects regardless of the spell slot used to cast them.

Once a warlock has spent their spell slots — at ninth level, once they have cast two spells — they can't cast any more spells (again, setting aside cantrips) until they've recovered their spell slots. Most spellcasters need a long rest (eight hours) to recover spell slots, but again warlocks are different, and recover all of their slots after a short rest (one hour).

To sum up, for a warlock:

• The spells you know dictate what you can use your spell slots for.
• You don't ever "use up" a known spell.
• You spend a spell slot to cast a spell you know. After that, the spell slot is gone, but you still know the spell.

Warlocks can swap known spells as they gain levels.

From the Known Spells of 1st Level and Higher section of the Warlock class description: (PHB, p. 107):

Additionally, when you gain a level in this class, you can choose one of the warlock spells you know and replace it with another spell from the warlock spell list, which also must be of a level for which you have spell slots.

For example when a warlock goes from 3rd to 4th level, their known spells goes from four to five (per the table on p. 107 of the PHB), so that warlock can add a new known spells of first or second level, and replace one of the first or second level spells they already know with another first or second level spell. This is how most warlocks will "upgrade" low-level known spells they don't need into more useful higher-level spells.

Answer 2

Here is one thing to keep in mind. Although they have access to cantrips and their mystic arcanum, Warlocks are considered to be the arcane half casters when put side by side with the likes of the sorcerer and wizard. Technically speaking, Warlocks do not even have the "spellcasting" feature that is granted to Paladins, Rangers, Eldritch Knights, and Arcane Tricksters.

When looked at in the context of the Paladin, with whom they share a number of flavor similarities, their spellcasting system begins to make more sense. The paladin casts spells from his deity by channeling the strength of his oath in the same way a warlock's patron allows her to access their magic via a pact. While a level 1 warlock may be very limited in their spellcasting ability it is because in context the pact between themselves and their patron is not yet solidified, while a 9th-level warlock has proven themselves to be worthy of additional power.

Look at the contrast between eldritch blast and witch bolt. At Level 1, the eldritch blast deals 1d10 and the witch bolt deals 1d12. At 5th level, the eldritch blast forms two separate 1d10 beams while the witch bolt, being cast at 3rd level now deals a single dosage of 3d12.

Unlike a wizard who may cast witch bolt at 1st, 2nd, or 3rd level, a warlock may only cast it at 3rd level. At 20th level, a warlock has effectively 4 5th-level spell slots and 1 each of 6th through 9th, while a wizard has 3 5th-level slots, 2 6th- and 7th-level slots each and 1 8th- and 9th-level slot.

Answer 3

"I'm a bit more of a visual learner so instead of someone just explaining what a spell slot is, please add a couple examples."

Perhaps looking at this from a different perspective will allow you to capitalize on your visual learning preference:

Consider a .38 caliber six shooter. A revolver that has six chambers for bullets. For this example we'll say all six chambers are the equivalent of six level one spells. Now, after you fire six times (spells or bullets) you're finished. [Unless you throw the gun at your target like in an old black & white TV show.]

• Larger spells:

Imagine/visualize the same revolver with four more chambers added on that are slightly larger than the original six chambers. To give you a total of ten chambers. Let's call them .44 caliber or second level spell slots. [Yes I know peculiar looking gun to say the least!]

Being .44 caliber, they are larger and "won't fit" into the. 38 caliber chambers. But, oddly enough the .38 caliber bullets do fit into the .44 caliber chambers.

• The benefits:

Here's the wondrous part: you are not only able to fire a .38 caliber bullet (read first level spell) in a .44 caliber chamber (read second level spell slot) it works better then if you used a .38 caliber chamber (read first level spell slot) as the spell does more.

I hope this analogy helps you visualize how spell slots work and how they are forwards compatible but not backwards compatible. Explaining it this way to my new players allowed them to fully understand the concept.