"The cat is eating a mouse."
Translation:Кошка ест мышь.
There are three types of nouns: first declension (masculine and neuter), second declension (feminine) and third declension (feminine softsgin nouns).
In the accustive case:
First declension nouns only change if they are animate (so, while 'мост' would stay the same, 'Брат' would change because that is a living being.)
Second declension nouns ALWAYS change. (Пицца becomes пиццу, and so on).
Third declension nouns, unlike second declension, do NOT change. In fact, they stay the exact same in the accustive case as they are in the nominative (дочь, мочь, мышь, etc. all end with a "zero ending" in the nominative and accusitive cases.
I guess it depends on a region. I always considered "кушать" to be a childish and pretty flowery word and I don't use it unless I'm talking to a child or being sarcastic, but after some conversations with other Russians here on Duolingo, I've learned that in some parts of Russia "кушать" is more common word and is seen as a preferable choice compared to "есть".