Learn Croatian: Život je zanimljiv.

Croatian doesn't use articles like "the" or "a" in English. Depending on the context, život can mean "life", "a life", or "the life". Here, it just translates directly to "life".

How do we know that život is a masculine noun?

In Croatian, you can usually tell a noun's gender by its last letter. Because život ends in a consonant (the letter t), it is a masculine noun. If it ended in -a, it would typically be feminine.

Why does zanimljiv end in a consonant here instead of -a or -o?

Adjectives in Croatian have to match the gender of the noun they describe. Since život is masculine, the adjective zanimljiv (interesting) takes its masculine form, which usually ends in a consonant. If the noun were feminine, it would be zanimljiva.

Is je the only way to say "is"?

Je is the most common, everyday way to say "is" (it's the short form of the verb "to be"). There is also a longer form, jest, which is used for emphasis, but you will mostly see and use je in normal sentences like this one.