The best replacement for happen is happened.

The best alternatives to happen are rated as following:
★★★★★  happened
★★★  occur
★★  happens
★  what happened
★  are they caused
  make excuses
  what will probably happen
  you can make things happen
  was it a dream
  threatening to happen
  make it happened
  just happened
  make them happen

Your alternative for happen is not listed? Suggest!
Speaking about happen, one probably shouldn't forget about enzymes.

Other search terms related to happen and their ratings are:
★★★★★ enzymes
★★★ sun
★★ ozone layer
★★ photosynthesis
★ bees
★ friction
★ gravity
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Google Trends

Scale is based on the average worldwide traffic of happen in all years. Click on graph for more.
happen
happened
occur