Warlock Multiclass Options: A Complete DnD 5e Guide
In D&D 5e (2014): Warlock Pact Magic spell slots are separate from the standard multiclass spell slot pool. You cannot use your Warlock slots to cast spells from other classes, and vice versa.
spellcasting - Can I counter an opponent's attempt to counter my own spell (while casting with spell slots only)? - Role-playing Games Stack Exchange
In the 2024 update to D&D 5e, the rule limiting the casting of multiple spells per turn has changed:
One Spell with a Spell Slot per Turn
On a turn, you can expend only one spell slot to cast a
Wizard Spell Slots 5e D&D + Spellbooks Explained!
To answer such burning questions you’ll need an understanding of spell slots in D&D 5e. ... For a deeper dive into Wizard spell slots, see our article – Wizard Spell Slots 5e + Spellbooks Explained!
D&D 5E - Artificer Alchemist / Warlock = Tons of Elixirs?
Character optimization guide for the DnD 5e Artificer's spell list.