Je m’assois sur le banc au bord de la mer.

Because the verb here is s’asseoir, which is a reflexive verb meaning to sit down.

• Je m’assois = I sit down
• J’assois quelqu’un = I seat someone

So the m’ means the action comes back to the subject: I seat myself / I sit down.

This is called elision. In French, me becomes m’ before a vowel or silent h to make pronunciation smoother.

• me + assois → m’assois

The same thing happens in many other forms:

• je m’appelle
• tu t’habilles
• il s’assied

No. S’asseoir is an irregular verb.

In fact, French allows more than one present-tense pattern for this verb. You may see:

• je m’assois
• je m’assieds

Both are accepted. The sentence uses je m’assois, which is completely correct.

That means learners should treat this as a verb to memorize, not one to build by a simple regular pattern.

Most naturally, Je m’assois describes the action of sitting down.

So it is usually closest to:

• I sit down
• I’m sitting down

If you want to describe the state of already being seated, French often uses:

• Je suis assis (for a man)
• Je suis assise (for a woman)

So the contrast is roughly:

• Je m’assois = I am taking a seat
• Je suis assis / assise = I am seated

Sur means on or onto, and a bench is understood as a surface you sit on.

So:

• sur le banc = on the bench

French would not normally use dans here, because you are not inside the bench.

Le banc means the bench, so it suggests a specific or identifiable bench.

• sur le banc = on the bench
• sur un banc = on a bench

French article choice works much like English here. If the speaker means a particular bench—perhaps one already known in the situation—le banc makes sense.

Because au is the automatic contraction of à + le.

• à + le → au

So:

• au bord = at the edge / by the edge

This contraction is required in standard French.

Similar contractions:

• à + les → aux
• de + le → du

Because mer is a feminine noun in French:

• la mer = the sea

After de, the article stays:

• de la mer = of the sea

You would use du only with a masculine singular noun:

• du jardin = of the garden
• but de la mer = of the sea

Literally, it means at the edge of the sea. In natural English, that is often:

• by the sea
• at the seaside
• on the seafront (depending on context)

So le banc au bord de la mer is the bench by the sea.

This expression is a little more vivid than just près de la mer (near the sea), because au bord de suggests being right by the water.

A rough pronunciation for an English speaker is:

zhuh ma-SWAH syur luh bahn oh bor duh lah mehr

A few useful pronunciation notes:

• Je sounds like zhuh
• m’assois ends in -swa, and the final s is silent
• banc has a nasal vowel; the final c is silent
• au sounds like oh
• bord is pronounced roughly bor; the final d is silent
• mer sounds like mehr

So the flow is:

Je m’assois sur le banc au bord de la mer.

Yes. French can move that phrase for emphasis or style.

For example:

• Je m’assois sur le banc au bord de la mer.
• Au bord de la mer, je m’assois sur le banc.

Both are grammatical. The original version is a very natural, neutral word order. The second puts more emphasis on where the action happens.