to happen
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to happen
To occur or happen, usually in connection with an unexpected, often negative or unusual event.
Die Polizei befragt Zeugen, um herauszufinden, was genau vorgefallen ist.
The police are questioning witnesses to find out exactly what happened.
'Vorfallen' (to happen) is a separable verb. In a main clause, the prefix 'vor' goes at the end, while in a subordinate clause it remains attached to the verb.
Gestern Abend ist hier etwas Seltsames vorgefallen.
Something strange happened here last night.
Since 'vorfallen' (to happen) describes an event or a change, the perfect tense is always formed with the auxiliary verb 'sein' (to be), not with 'haben' (to have).
Was ist denn hier vorgefallen?
What on earth happened here?
The verb 'vorfallen' (to happen) is usually used for unexpected, unusual, or negative events. It often has a more serious undertone than 'passieren' (to happen).
Nach dem Streit fragte sie ihn, ob so etwas schon einmal vorgefallen sei.
After the argument, she asked him whether something like that had ever happened before.
to happen / pass
'passieren' and 'vorfallen' can both mean that something happens. 'passieren' is neutral and very general. It can refer to normal, small, or even serious events. 'vorfallen' is used more for an unexpected, unusual, or unpleasant event. It often sounds as if the incident needs to be clarified or reported on. If you are asking generally what happened, 'passieren' fits. If it is about a problem, an argument, or a special situation, 'vorfallen' is often more precise.
Nach dem Unterricht ist auf dem Schulhof etwas Ernstes vorgefallen.
After class, something serious happened in the schoolyard.
Keine Sorge, beim Kochen kann so ein Fehler passieren.
Don't worry, that kind of mistake can happen when cooking.