La mouette vole au-dessus de la mer.

French usually requires an article before a singular count noun in a normal sentence. So la mouette is the natural way to say the seagull.

Also, la shows that mouette is:

• singular
• feminine
• definite (the, not a)

If you wanted a seagull, you would say une mouette.

You know it is feminine because the sentence uses la.

In French, every noun has a grammatical gender, and mouette is a feminine noun. There is not always a logical reason for gender, so it usually has to be learned with the noun itself:

• la mouette
• une mouette

A helpful pattern is that many nouns ending in -ette are feminine, though this is not a rule you can trust 100% in every case.

Vole is the third-person singular present tense form of voler.

Here is the relevant pattern:

• je vole
• tu voles
• il / elle vole
• nous volons
• vous volez
• ils / elles volent

Because la mouette is singular and equivalent to elle, the verb form is vole.

Not for gender.

In the present tense, French verbs agree with the subject in person and number, but not in gender. So:

• La mouette vole
• Le goéland vole

Both use vole, even though one noun is feminine and the other is masculine.

Gender agreement happens with things like articles and many adjectives, but not with this present-tense verb form.

Yes, voler can mean both to fly and to steal.

French often has words with more than one meaning, just like English. The context makes the meaning clear. In this sentence, with la mouette and au-dessus de la mer, the only sensible meaning is to fly.

For example:

• L’oiseau vole = the bird flies
• Le voleur vole un sac = the thief steals a bag

French often uses the simple present where English might use either:

• flies
• is flying

So La mouette vole can mean:

• The seagull flies
• The seagull is flying

French does have ways to emphasize an action in progress, such as être en train de, but it is not necessary here.

Au-dessus de means above or over.

It is a fixed expression used to show that something is at a higher position than something else:

• au-dessus de la mer = above the sea

A useful way to learn it is as one whole chunk:

• au-dessus de + noun

For example:

• au-dessus de la table = above the table
• au-dessus de la ville = above the city

Because au-dessus de is the normal pattern when it is followed by a noun.

So you say:

• au-dessus de la mer
• au-dessus du village
• au-dessus des nuages

Think of au-dessus de as a complete prepositional expression, not as separate pieces you can freely rearrange.

Because mer needs its article here, and mer is feminine singular.

So:

• de + la mer = de la mer

You do not drop the article in this sentence. French normally keeps the article with nouns like la mer, la table, le ciel, etc., unless a different grammar rule removes or changes it.

Compare:

• au-dessus de la mer
• au-dessus du port = above the port
• au-dessus des vagues = above the waves

Because au-dessus de and sur are not the same.

• au-dessus de = above, over
• sur = on, on top of, sometimes over depending on context

Au-dessus de usually suggests a position higher than something, often without contact.
Sur often suggests being on a surface or directly over it in a more immediate way.

So for a seagull flying in the air, au-dessus de la mer is very natural.

A simple pronunciation guide is:

la mwet vol o-duh-sy duh la mair

More carefully:

• La → la
• mouette → mwet
• vole → vol
• au-dessus → o də-sy
• de la mer → də la mɛʁ

A few notes:

• mouette is one syllable after mw- starts: mwet
• au sounds like o
• the final r in mer is the French r

Because au-dessus is a fixed compound expression, and in standard spelling it is written with a hyphen.

You should learn it as:

• au-dessus
• au-dessous

The hyphen is simply part of the correct written form.

It would become:

Les mouettes volent au-dessus de la mer.

Changes:

• la → les
• mouette → mouettes
• vole → volent

Notice that the pronunciation of volent is still basically vol in normal speech, even though the spelling changes.