Learn Russian: Этот ящик для бананов.

This is the genitive plural form of the word банан (banana). Since банан is a masculine noun ending in a hard consonant, we add -ов to make it plural in the genitive case.

Why is the genitive case used for bananas here?

The preposition для (meaning "for" or "intended for") always requires the noun that follows it to be in the genitive case.

Where is the verb "is" in this sentence?

In Russian, the present tense of the verb "to be" is almost always left out. The sentence translates literally as "This box for bananas," which is the standard and natural way to say "This box is for bananas."

Why do we use Этот instead of Это?

The word for "this" must match the gender of the noun it describes. Ящик (box) is a masculine noun, so we must use the masculine form Этот, rather than the neuter form Это or the feminine form Эта.