The event is cancelable.

If the event is canceled, the farmland won't turn to dirt

The event does not have a result.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.

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import crafttweaker.api.event.block.MCFarmlandTrampleEvent;

MCFarmlandTrampleEvent extends MCBlockEvent. That means all methods available in MCBlockEvent are also available in MCFarmlandTrampleEvent

Return Type: MCEntity

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MCFarmlandTrampleEvent.getEntity() as MCEntity
myMCFarmlandTrampleEvent.getEntity();

Return Type: float

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MCFarmlandTrampleEvent.getFallDistance() as float
myMCFarmlandTrampleEvent.getFallDistance();