happened

English as a Second Language English Vocabulary, Grammar and Idioms
1
  1. That’s a long time in the past!
  2. That happened a long time in the past!
  3. That was happened a long time in the past!
  4. That incident happened a long time in the past!
  5. That incident happened a long time before.
    Please correct the grammatical error in these sentences.
2
  1. That’s a long time in the past!
  2. That happened a long time in the past!
  3. That was happen[ing] a long time in the past!
  4. That incident happened a long time in the past!
  5. That incident happened a long time before.

All of these are somewhat context-dependent, but apart from #3 they could all be correct.