adverb
otherwise, besides
A2
'sonst' mainly means 'otherwise' (indicating consequence or alternative) and 'besides' (in addition). It's an adverb used to introduce alternatives or additional remarks. No specific preposition or reflexivity. Common in conditional sentences: 'Mach das, sonst...'. Neutral register; frequent in spoken and written German.
Examples
Beeil dich, sonst verpasst du den Zug.
Hurry up, otherwise you'll miss the train.
Er legte den Schlüssel gut sichtbar hin, damit seine Frau ihn sonst nicht suchen musste.
He placed the key in a visible spot so that his wife would not have to search for it otherwise.
Ich habe keine Zeit, und sonst habe ich viel Arbeit.
I don't have time; besides, I have a lot of work.
Details
Mnemonics
Imagine a sign with two arrows: one labeled 'sonst' pointing to an alternative path
sounds like 'sonst' ~ 'once' with an s — think 'if not, then once?'.
Notes
Common adverb with several uses: to indicate an alternative (otherwise) or to add a secondary remark (besides).