The best replacement for happened is happen.

The best alternatives to happened are rated as following:
★★★★★  happen
★★★★★  has happened
★★  what happen
★★  happened
★★  occurred
★★  what happened
★★  happend
★  if you happen
★  made it happen
★  had happened
  what ever happened

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Speaking about happened, one probably shouldn't forget about tarp.

Other search terms related to happened and their ratings are:
★★★★★ tarp
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happened
happen
has happened