Why (the) hen when there wasnt Ta slepice or ty slepice
Because it can be a specific hen we are talking about. Especially when we have a specific "tu kočku/the cat". Remember, Czech does not have articles/determiners. only demonstrative pronouns and they follow different rules.
Plural for "slepice" is "slepice."
Plural for "vidí" is "vidí." So why can't this be: "The hens don't see the cat" ?
Right you are... edited
It could, yes.