"A lion is not a tiger."
Translation:Lew to nie tygrys.
Cases. "tygrys" is Nominative, "tygrysem" is Instrumental.
For more knowledge about cases, check this post for topics about cases: https://www.duolingo.com/comment/16296174
For more knowledge about the usage of these two specific cases, in sentences similar to this one, check this post, especially Part 1: https://www.duolingo.com/comment/16373167
And there are two possible answers here: "Lew nie jest tygrysem" and "Lew to nie tygrys".
Generally Polish has free word order when it comes to nearly every aspect of the speech, sentence, etc. The only thing that you cannot do is to separate 'nie + verb' while forming a negation; apart from that everything is permitted (no other rules come to my mind at the moment).