Cases. This is a simple "X is Y" sentence, X is in Nominative (the basic form), and the basic form of masculine "this" is "ten".

"tego" is Genitive, and Genitive is used in many cases, some of the most common ones are possession (To jest jedzenie tego lwa = This is this lions' food) or with negated verbs (Nie lubię tego lwa = I don't like this lion).

1. I'm afraid those are not Polish words. The special character Ł, used here, is not a variant of T. As you see in the capital letter, it's a variant of L.

2. The adjective has to match the noun it describes in terms of grammar. Just like in Spanish.

So "zły lew" = el león malo = a bad lion. It's masculine.

"zła krowa" = la vaca mala = a bad cow. It's feminine.

"złe" is neuter, there is no neuter gender in Spanish.

And I chose animals that have the same gender in both these languages, but often it will be different. The gender is generally dependent on the ending of the noun. Masculine nouns mostly end with a consonant, feminine nouns mostly end with -a, neuter nouns end with -o, -ę or -um. There are exceptions though.

It can be 'angry', but it's not the primary meaning.

Those sentences teaching "zły" are... well, bad. The word seems to be super-basic, but it's in fact not that easy in usage and should be rather taught later in the course.