BE动词和助动词在这句话中的用法疑问.

why isn't Mrs.lee's cat catching the mice?
catching是动词,"is catching" 是现在进行时句型.
如,I am watching TV. 我正在看电视.
why dose't Mrs.lee's cat catching the mice?
这句话是错的,应该改为：why doesn't Mrs. Lee's cat catch the mice? 要用catch 原型动词.这是一般现在时句型.