Το φεγγάρι πάνω από τη θάλασσα είναι πολύ ωραίο απόψε.

Because φεγγάρι is a neuter singular noun, so it takes the neuter singular definite article το.

• το φεγγάρι = the moon
• masculine nouns often take ο
• feminine nouns often take η
• neuter nouns often take το

So the article has to match the noun’s gender and number.

Yes. Το φεγγάρι is the common everyday Greek word for the moon.

You may also see η σελήνη, which is a more formal or scientific word, closer to Selene / the moon in a more literary or technical sense.

So in normal speech:

• το φεγγάρι = the usual everyday word
• η σελήνη = more formal, poetic, or scientific

Πάνω από is a very common Greek expression meaning above, over, or up above.

It is made of:

• πάνω = up / on top / above
• από = from, but in this expression it helps form the meaning above / over

So:

• πάνω από τη θάλασσα = above the sea

You should learn πάνω από as a unit here, not by translating each word separately every time.

Because after πάνω από, the noun goes in the accusative case, not the nominative.

So the noun θάλασσα changes like this:

• nominative: η θάλασσα = the sea
• accusative: τη θάλασσα = the sea

In this sentence:

• πάνω από τη θάλασσα = above the sea

This is very common in Greek: prepositions often require a particular case, and από takes the accusative.

Both are related to the same feminine accusative article.

• full form: την
• often shortened before a consonant: τη

So both of these can exist in Greek, depending on style, pronunciation, and the sound that follows. In everyday writing, τη θάλασσα is completely normal.

You will also often hear την before vowels or certain consonants more clearly in careful speech.

Because the adjective must agree with the noun it describes.

The noun is:

• το φεγγάρι = neuter singular

So the adjective must also be neuter singular:

• masculine: ωραίος
• feminine: ωραία
• neuter: ωραίο

That is why the sentence says:

• Το φεγγάρι ... είναι πολύ ωραίο
  = The moon ... is very beautiful / nice

All of those are possible depending on context.

Ωραίος / ωραία / ωραίο is a very common Greek adjective that can mean:

• beautiful
• nice
• lovely
• attractive
• wonderful

In this sentence, πολύ ωραίο most naturally means something like:

• very beautiful
• very nice
• lovely

So it is a broad, flexible adjective.

Here είναι means is.

• είναι = is / are

In this sentence, it links the subject to the description:

• Το φεγγάρι ... είναι πολύ ωραίο
  = The moon ... is very beautiful

Greek often uses είναι in the same kind of way English uses is/are with adjectives.

A useful point: είναι is the same form for he/she/it is and they are, so context tells you which is meant.

Απόψε means tonight.

It refers specifically to this evening / tonight, and it is a very common adverb.

So:

• απόψε = tonight

Compare:

• σήμερα = today
• σήμερα το βράδυ = this evening / tonight
• απόψε = tonight

In many cases, απόψε is the more natural single-word choice.

Greek word order is more flexible than English word order. Putting απόψε at the end sounds natural and gives a smooth finish to the sentence.

So:

• Το φεγγάρι πάνω από τη θάλασσα είναι πολύ ωραίο απόψε.

is a normal Greek sentence.

You could move απόψε earlier for emphasis, but the original order is very natural.

Yes. It tells you which moon / where the moon is in the speaker’s view.

So the structure is roughly:

• Το φεγγάρι = the moon
• πάνω από τη θάλασσα = above the sea
• είναι πολύ ωραίο = is very beautiful
• απόψε = tonight

In other words, the phrase πάνω από τη θάλασσα acts like a descriptive location phrase attached to the moon.

Yes, that is a very good literal translation.

Depending on style, it could also be understood as:

• The moon over the sea is very beautiful tonight.
• The moon above the sea is very beautiful tonight.

Both above and over work well here. Over the sea often sounds a bit more natural in English, while πάνω από is the Greek phrase used.

A simple learner-friendly pronunciation is:

• to feg-GA-ri PA-no a-PO ti THA-la-sa I-ne po-LI o-RE-o a-POP-se

A few notes:

• γγ in φεγγάρι sounds like an ng-type sound
• θ in θάλασσα is like th in think
• χ is not in this sentence, but Greek learners often expect it
• stress matters, and the written accent marks show you where it goes:
  • φεγγάρι
  • πάνω
  • από
  • θάλασσα
  • πολύ
  • ωραίο
  • απόψε

Yes. That would still be a correct sentence:

• Το φεγγάρι είναι πολύ ωραίο απόψε.
• The moon is very beautiful tonight.

Adding πάνω από τη θάλασσα simply gives extra imagery and makes the sentence more specific:

• The moon above the sea is very beautiful tonight.

So the longer sentence is more descriptive, but the shorter one is perfectly natural too.

The sentence follows this pattern:

• subject: Το φεγγάρι
• descriptive phrase: πάνω από τη θάλασσα
• verb: είναι
• adverb + adjective: πολύ ωραίο
• time adverb: απόψε

So a useful breakdown is:

• Το φεγγάρι = subject
• πάνω από τη θάλασσα = location/descriptive phrase
• είναι = is
• πολύ ωραίο = very beautiful
• απόψε = tonight

This is a very typical and useful Greek sentence structure.