Paul et Marie se promènent au bord de la mer.

Because se promener is a pronominal (reflexive-style) verb in French. It is normally used with a reflexive pronoun:

• je me promène
• tu te promènes
• il/elle se promène
• nous nous promenons
• vous vous promenez
• ils/elles se promènent

In this sentence, the subject is Paul et Marie, which is they, so French uses se promènent.

Even though English often just says Paul and Marie are walking or are taking a walk, French usually keeps the pronoun with this verb.

Se promener usually means to take a walk, to go for a walk, or to stroll.

So Paul et Marie se promènent suggests that Paul and Marie are:

• walking around
• strolling
• taking a walk

It often has a relaxed feeling, not just walking to get somewhere quickly.

Because the subject is Paul et Marie, which means Paul and Marie. Two people = third person plural.

So the verb must match that subject:

• Paul se promène = Paul is taking a walk
• Marie se promène = Marie is taking a walk
• Paul et Marie se promènent = Paul and Marie are taking a walk

The ending -ent marks the ils/elles form in the present tense.

The form se promènent is the present tense of se promener for ils/elles.

So the sentence can mean:

• Paul and Marie are taking a walk by the seaside
• Paul and Marie take a walk by the seaside
• depending on context, sometimes even Paul and Marie are walking by the sea

French present tense often covers both the English simple present and present continuous.

Because à + le contracts to au in French.

So:

• à + le = au
• à + les = aux

Here, bord is masculine: le bord = the edge / shore / side

So:

• à le bord → au bord

That is why French says au bord de la mer.

Literally, it means at the edge of the sea.

Breakdown:

• au = at the / to the
• bord = edge, side, shore
• de la mer = of the sea

So the whole phrase means:

• by the sea
• at the seaside
• along the seashore

In natural English, by the sea or along the seaside is usually the best translation.

Because French normally uses an article with nouns in many places where English does not.

Here:

• la mer = the sea
• au bord de la mer = at the edge of the sea / by the sea

French usually prefers de + article + noun here, not bare de mer.

So au bord de la mer is the normal expression.

Because French does not need a separate word like are to make the present continuous in this sentence.

English says:

• Paul and Marie are walking

French simply uses the present tense:

• Paul et Marie se promènent

So one French verb form often corresponds to are walking, walk, or go for a walk, depending on context.

If French wants to strongly emphasize in the middle of doing something, it can use another structure such as être en train de, but that is not necessary here.

Not exactly.

It uses reflexive pronouns, but in many cases it is better to think of it as a set pronominal verb rather than literally to walk oneself.

For example:

• se laver = to wash oneself
• se promener = to take a walk / stroll

So the se is required by the verb pattern, but you should not translate it word-for-word as themselves here.

In standard French, the final -ent of ils/elles verbs is usually not pronounced.

So promènent sounds essentially like promène.

That means:

• il se promène
• ils se promènent

sound very similar, and sometimes identical in speech. You usually tell the difference from the subject:

• il
• Paul et Marie
• ils / elles

Because se becomes s' only before a vowel sound or a silent h.

Examples:

• Ils s'amusent
• Elle s'habille

But here the next word is promènent, which begins with a pronounced consonant p, so French keeps se:

• Paul et Marie se promènent

Yes, but the meaning changes a little.

• se promènent = are strolling / taking a walk / going for a walk
• marchent = are walking

Marcher is more neutral and focuses on the physical action of walking.
Se promener often suggests a leisurely walk, for pleasure.

So both can work, but se promènent sounds more natural if the idea is a pleasant walk by the sea.

In standard written French, you normally do not repeat the subject pronoun when the noun subject is already there.

So the correct sentence is:

• Paul et Marie se promènent...

Not:

• Paul et Marie ils se promènent...

In informal spoken French, people sometimes do use a repeated pronoun for emphasis or natural speech rhythm, but learners should treat that as a separate spoken pattern, not the normal standard structure.

Yes.

The French present tense can describe:

• something happening right now
• something generally true
• something that happens regularly

So depending on context, Paul et Marie se promènent au bord de la mer could mean:

• Paul and Marie are taking a walk by the sea
• Paul and Marie walk by the sea
• Paul and Marie go for walks by the sea

Without context, the French form itself allows all of these possibilities.