What does "as if you were a single-classed member of that class" mean for spells known and spell slots?

Question

Introduction

I am currently building a multiclass spellcaster, and came across this question:

Can a multiclass Wizard copy any Wizard spell they find into their spellbook?

This discussion appears to hinge on the phrase

as if you were a single-classed member of that class.

from p. 164 of the PHB. However, what that means is never discussed in terms of RAW. The twittersphere has produced this tweet, alluded to by @Cambio in his answer to the above question, but twitter is not a platform for discussion, and so justification was not forthcoming. Furthermore, tweets are not RAW.

So, to my specific question:

Do you determine spell slots for individual classes throughout character leveling up, then recalculate at the end (as "as if you were a single-classed member of that class" implies)? Or do you only consider one pool of spell slots as explicitly and specifically outlined on p. 164 of the PHB throughout the entire process of leveling up?

As an example, an answer would indicate how the flow of Wizard 4 dipping a level into Sorcerer at character level 5 would go:

1. Start at Wizard 4. Add 1 level to character as Sorcerer.
2. Look at number of spells known on table of p. 100.
3. Look at spell slots on table of p. 100 for a level 1 Sorcerer.
4. Determine the number of spells known is 2 and level of spell slots is 1.
5. Learn two level 1 spells based on the spells known of 1st level and higher section on page 101 for the Sorcerer class.
6. Look at the actual spell slots of the character from the table on p. 165.

Or

1. Start at Wizard 4. Add 1 level to character as Sorcerer.
2. Look at number of spells known on table of p. 100.
3. Look at spell slots on table of p. 165.
4. Determine the number of spells known is 2 and highest level of spell slots is 3.
5. Learn two spells of levels 1, 2, or 3 based on the spells known of 1st level and higher section on page 101 for the Sorcerer class.

Answer 1

Easier one first.

Spell Slots

You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, and half your levels (rounded down) in the paladin and ranger classes. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table. Basic Rules

Wizard 4, Sorcerer 1? Look for level 5 in Multiclass Spellcaster table.

4 level 1 slots, 3 level 2 slots, 2 level 3 slots.

Spells Known and Prepared

You determine what spells you know and can prepare for each class individually, [...]

Consult Wizard Spellcasting table for level 4 Wizard, and Sorcerer Spellcasting table for level 1 Sorcerer.

4 cantrips, 4 1st-level spells, 3 2nd-level spells (from Wizard 4)

4 cantrips, 2 1st-level spells (because you only a Sorcerer level 1)

In summary, you have 8 cantrips, 6 1st-level spells, 3 2nd-level spells. You have 4 slots level 1, 3 slots level 2, 2 slots level 3.

Note that though you have level 3 spell slots, you don't actually know any 3rd-level spell. You can use these slots to cast your lower level spells. Spells cast using higher spell slots may have stronger effect, if their description include "Higher casting" section.

If you have more than one spellcasting class, this table might give you spell slots of a level that is higher than the spells you know or can prepare. You can use those slots, but only to cast your lower-level spells. If a lower-level spell that you cast, like burning hands, has an enhanced effect when cast using a higher-level slot, you can use the enhanced effect, even though you don't have any spells of that higher level. Basic Rules

Additionally, your upcast spell level is the level of spell slot you use to cast that spell. This is not optional.

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. Basic Rules

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as if you were a single-classed member of that class.

This phrase is to clarify that for wizard's spell known and prepared, you use only your wizard level, not the combined level of wizard and sorcerer (and other class with Spellcasting feature). Likewise, you only use your sorcerer level to determine your sorcerer's spell known and prepared.

Answer 2

The first procedure is the correct one.

Multiclass characters get one pool of spell slots using the table on page 165 of the Player's Handbook.

However, the spells that can be cast using those slots are determined by considering each subclass separately. This is what “as if you were a single-classed member of that class” means: in our specific example, "as if you were a 4th-level Wizard" and "as if you were a 1st-level Sorcerer", determined separately.

Note that your to-hit bonus and your save DC may also be different for your Wizard spells (which use Intelligence) and your Sorcerer spells (which use Charisma).

Perhaps your confusion arises from having played past editions? In 5E you do not prepare spells slot by slot like in many previous editions of the game; instead you have a pool of known/prepared spells and a pool of slots; slots work very much like "mana" or "magic points", if you will.

A Wizard 4 / Sorcerer 1 therefore has 4 first-level slots, 3 second-level slots and 2 third-level slots. However, they cannot prepare any "native" third-level spells like Fireball, because neither a 4th-level Wizard nor a 1st-level Sorcerer could cast a third-level spell. They won't even be able to copy such a spell in their spellbook, for the reasons detailed in the question you linked. So what can our Wiz4/Sor1 do with their third-level slots? They can only use them to cast lower level spells; they do get to enhance these spells if such an option is offered in their description: for instance, if they cast Magic Missile with a third-level slot, they would create five missiles instead of three.

Answer 3

Your first method is the correct one

To determine what Sorcerer spells you can learn, you look only at Sorcerer class

The rules for multiclassing tell us what we have to do to determine what spells a character can know:

You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.

That means, when putting your 5 level into Sorcerer 1, you only consider one level of Sorcerer for the purposes of determining what spells you can learn. The rule for a single-classed Sorcerer says:

The Spells Known column of the Sorcerer table shows when you learn more sorcerer spells of your choice. Each of these spells must be of a level for which you have spell slots.

The table tells you that a normal sorcerer has 2 spells known and 2 first level slots.

This means that you must use one of these available slots to pick the spells you learn.

Spell slots available to use depend on all your spellcasting classes together

Looking at the multiclassed spellcasting table will show you what spell slots your new level 5 character has available to use to cast spells:

4 1st-level slots, 3 2nd-level slots, 2 3rd-level slots.

As stated above, these slots do not count for determining what spells you know or can prepare since these slots come from both classes and determining spells comes as if you were a single-class. It isn't a matter of order of operations; these slots simply never matter when you are figuring out what spells your character knows.

The rules even specifically says that when multiclassing you might end up having higher level slots than the level of spells you know:

If you have more than one spellcasting class, this table might give you spell slots of a level that is higher than the spells you know or can prepare. You can use those slots, but only to cast your lower-level spells.

And that is exactly what is happening here — even though you have 3rd-level slots, you don't know any 3rd-level spells.

Thus, your wizard/sorcerer will be able to learn 2 1st-level sorcerer spells upon adding that 1st level of sorcerer. However they will be able to cast those 1st level spells using up to a 3rd level slot.