noun
idea, notion, sudden idea
B1
Einfall: means 'idea', 'notion', often 'sudden idea'. Masculine noun (der Einfall), plural Einfälle (note umlaut). Use der (sing.) and die (pl.). Common verbs: einen Einfall haben (to have an idea). Regular declension except plural umlaut change; no special preposition. Colloquially.
Examples
Ihr spontaner Einfall, die Reise zu verschieben, rettete uns viel Geld.
Her sudden idea to postpone the trip saved us a lot of money.
Sein Einfall, das Projekt sofort zu stoppen, überraschte das ganze Team.
His notion to stop the project immediately surprised the whole team.
Der Vorschlag war ein guter Einfall, weil er Zeit und Geld sparen konnte.
The proposal was a good idea because it could save time and money.
Details
Mnemonics
Picture a light bulb that 'falls' into your brain from above when you get an Einfall.
Sounds like 'in-fall' — imagine an idea falling into your head.
Der Einfall — imagine a male ('der') inventor having the idea, wearing a hat.
Notes
Einfall often refers to a single, often sudden idea or notion (compare to 'Idee', which can be more neutral). Common plural: Einfälle. Watch out for expressions like 'auf einen Einfall kommen' (to come up with an idea).