verb
to stand out, to be noticeable
B1
auffallen: separable verb (auf|fallen) meaning 'to stand out' or 'to be noticeable'. Intransitive; commonly used with a dative experiencer (es fällt mir auf). Auxiliary sein for perfect (ist aufgefallen). Strong/irregular: Präsens fäll-; Präteritum fiel; Partizip II aufgefallen. Passive uncommon. Use note: cannot take accusative object.
Examples
In der Gruppe fällt sein rotes Hemd sofort auf.
In the group his red shirt immediately stands out.
Sein Verhalten fiel mir sofort auf.
His behavior immediately caught my attention.
Mir fällt auf, dass du heute glücklich bist.
I notice that you're happy today.
Details
Mnemonics
Imagine a bright neon sign that 'falls off' a grey wall and suddenly everyone notices it.
Think 'off' + 'fallen' — something that 'falls off' will draw attention (stands out).
Notes
A separable verb: in main clauses the prefix 'auf' separates (e.g., 'es fällt mir auf'). Its perfect uses the auxiliary 'sein' (ist aufgefallen). | Intransitive verb; passive voice conjugation blocks are not applicable.