"I know a dog."
Because it will change the meaning: 'ten pies' indicates, that you are talking about some specific dog, and that would require 'the/this dog' in English.
I know we're halfway through this exercise. But I just realized that there is another word for "to know." I'll probably find the lesson for this as soon as I finish. But in case there isn't one, what is the difference between wiedzieć and znać?
What is the difference between psem and psa? Which case is poem and when do I use it? edited
"poem" I suppose you meant "psem"?
It's Instrumental. Like "Rex jest psem, a nie kotem" (Rex is a dog, and not a cat).