La lune est pleine.

In French, a singular common noun usually needs an article. So la lune is the normal way to say the moon.

French uses articles more often than English does, especially with general things and natural objects. Saying just lune by itself would usually sound incomplete here.

Because lune is a feminine noun.

In French, nouns have grammatical gender, and lune is feminine, so it takes la:

• la lune = the moon

If it were a masculine noun, it would usually take le instead.

Est is the he/she/it is form of the verb être, which means to be.

So:

• la lune = the moon
• est = is
• pleine = full

Because la lune is singular, French uses the singular form est.

Because French adjectives usually agree with the noun they describe.

Since lune is feminine singular, plein changes to pleine.

Compare:

• Le verre est plein = The glass is full
• La lune est pleine = The moon is full

So the extra e shows feminine agreement.

Yes. Pleine literally means full.

In this sentence, it is the normal French word used to describe the moon when it is in the full moon phase. So it is both literal and natural.

Both are correct, but they are doing different jobs.

• La lune est pleine = a complete sentence, meaning The moon is full
• La pleine lune = a noun phrase, meaning the full moon

So:

• La lune est pleine ce soir. = The moon is full tonight.
• J’aime la pleine lune. = I like the full moon.

A very common related sentence is C’est la pleine lune, which means It’s a full moon or It’s the full moon phase.

Not naturally in this context.

For the moon phase, French normally uses pleine, not complète.
Complète means complete, but that is not the usual word for describing a full moon.

So the idiomatic choice is:

• La lune est pleine

Yes, very close.

French uses:

• subject
  • verb
    • adjective

So:

• La lune = subject
• est = verb
• pleine = adjective

This matches English very neatly:

• The moon
  • is
    • full

A rough English-friendly approximation is:

la lün eh plen

More precisely, in IPA:

[la lyn ɛ plɛn]

A few useful notes:

• u in lune is the French u, which does not sound exactly like English oo
• the final e in lune is silent
• the final t in est is not pronounced here
• the final e in pleine is also silent

In everyday French, lune is usually written with a lowercase l.

French uses capital letters less often than English. In scientific or astronomical writing, you may sometimes see la Lune when referring specifically to Earth’s moon as a celestial body, but in ordinary language la lune is completely normal.