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run over

Overrun is a verb.

It can mean to spread over or occupy (a place) in large numbers.

For example: If bin collections do not improve we could be overrun by rats.

It can also mean, to conquer or occupy a territory by force.

For example: The enemy had overrun the country during the first half of the war.

Run over is a phrasal verb.

It can mean to collide with, knock down, and even physically go over someone or something.

For example: The child was run over by the car.

It can also mean to go beyond a time limit.

For example: Presentations would often run over due to bad moderating.