geschehen (class 5 strong, third-person singular presentgeschieht, past tensegeschah, past participlegeschehen, past subjunctivegeschähe, auxiliarysein)
Only third-person forms of this verb are common. First and second-person forms are very rare, though not impossible. For example, one could say in poetic speech: O Unglück! Wenn du geschiehst, dann geschieh bald! (“O misfortune! If thou happenest, then happen soon!”)
In most regions, geschehen is rather formal, with passieren being the more normal word for “to happen”.
