Learn Russian: Зоопарк большой и красивый.

In Russian, the present tense of the verb "to be" (is, am, are) is simply skipped. So, Зоопарк большой literally translates to "Zoo big", but in English we naturally add "is" to make it "The zoo is big".

Why do the two adjectives have different endings (большой vs красивый) if they both describe the same word?

Both are standard masculine adjective endings. The difference comes down to spoken stress (emphasis). Because the stress in большой falls on the last syllable, it must take the -ой ending. The word красивый is stressed earlier in the word, so it takes the unstressed -ый ending.

How do we know it translates to "The zoo" and not "A zoo"?

Russian does not use articles like "a" or "the". You have to figure it out from the context of the conversation. Since this sentence is describing the traits of a specific zoo, "The zoo" is the most natural English translation.

How do we know that зоопарк is a masculine noun?

You can tell by looking at its last letter. In Russian, nouns that end in a consonant, like the к in зоопарк, are almost always masculine. Because the noun is masculine, the adjectives describing it must also be in their masculine forms.