Learn Russian: Море не близко, а далеко.

In Russian, the present tense of the verb "to be" (like "is" or "are") is almost always left out. You simply put the subject (море) and its description (не близко) side by side.

Why does this sentence use а for "but" instead of но?

Russian has different words for "but". You use а when contrasting two things, especially when correcting a negative statement with a positive one ("not this, but that"). Since we are saying the sea is not close but far, а is the exact word you need.

What gender is the word море (sea)?

It is a neuter noun. In Russian, nouns ending in -о or -е are usually neuter. Remembering this will help you match it with the correct adjective endings later, like if you wanted to say "the beautiful sea".

Why do the words близко (close) and далеко (far) end in -о?

These are adverbs, which are words that describe where or how something is. In Russian, many common adverbs end in -о. Unlike adjectives, adverbs don't change their endings to match the gender of a noun, so they will always stay близко and далеко here.