What is the past tense of ground?

Answer 1

It depends on the context.

'Ground' is already past tense. It is the past tense of 'grind'.

On the other hand, you can be 'grounded' for staying out past your curfew.

Answer 2

The past tense of "ground" is "ground" or "grinded," depending on the context. "Ground" is more commonly used as the past tense when referring to the action of grinding something down, while "grinded" may be used in more informal or dialectal contexts.