"A lion is a cat."

Translation: Lew jest kotem.

[cosmopolita61]

Is lew to kotem wrong?

[Vlad.Volskiy]

"Lew jest kotem" = "Lew to kot"

[currymia]

What is the difference between kot and kotem?

[Jellei]

"Kot" is Nominative, "kotem" is Instrumental. There are two ways to translate a sentence like "X is Y" when both X and Y are noun phrases (you will soon find explanations under some other sentences). One is "X + a form of 'być' + Y" in Instrumental (so: Lew jest kotem), and the other is simple "X to Y" with Y in Nominative, like here.

[Smurfetka3]

Why can't we say "to" instead of "jest". I would like to have some explanations please on the difference and when to use each one.

[Jellei]

We can. What exactly did you write? It's either "Lew jest kotem" or "Lew to kot", rarely "Lew to jest kot".

This post explains everything: https://forum.duolingo.com/comment/16373167

[NickFleck]

Why not Lew jest kotem? as with other examples

[alik1989]

That is the main answer.