The gerund is the word which contains 'ing' to it. It basically is added to things that we're doing right now or were doing....
For example:
I am eating
She was crying
The dog is barking
So, the answers are:
The cat was chasing a mouse through the house.
The horse was galloping down the road.
The cow was chewing the fresh hay.
Hope this helps chu
Have a great day
