verb
to find out
B1
feststellen: separable verb meaning 'to find out', 'to ascertain' or 'to determine'. Regular (weak) verb; auxiliary haben; past participle festgestellt. Separable in present and imperative (du stellst fest / stell fest!). Not reflexive. Often used for factual conclusions (Feststellen eines Fehlers) and in passive constructions (festgestellt werden).
Examples
Er stellte einen Fehler fest.
He noticed a mistake.
Der Techniker stellte das Problem nach einer Untersuchung fest.
The technician discovered the problem after an inspection.
Der Arzt stellte fest, dass das Medikament die Symptome nicht linderte.
The doctor found that the medicine did not relieve the symptoms.
Details
Mnemonics
Imagine pinning a note to a board to make something fixed and known — you 'pin it down' (fest = fixed, stellen = place).
Think 'find out' = 'find + still' -> make something fixed/clear: 'feststellen'.
Notes
A separable verb: in main clauses the prefix 'fest' moves to the end (ich stelle fest). Common in formal and spoken German.