"Is a lion not a cat?"
Translation:Czy lew to nie kot?
Well, the hints are for the whole course so they won't necessary work in every sentence.
"To lew, nie kot" (if you add the comma) means "This is a lion, not a cat". Quite a different sentence.
"to" isn't exactly "is", although it works like it. "Lew to kot" (A lion is a cat) is kinda like "A lion = a cat". And here it is definitely in the right place.