En haut de la colline, on voit la mer.

En haut de means at the top of, high up on, or up at the top of something.

So in en haut de la colline, the idea is that the viewpoint is high on the hill, especially near the top.

A useful contrast:

• en haut = up high / at the top
• en haut de la colline = at the top of the hill

So de is needed because you are specifying the thing whose top you mean.

Because colline is a feminine singular noun, so it takes la.

After de:

• de + le → du
• de + les → des
• de + la stays de la
• de + l’ stays de l’

So:

• de la colline = correct
• du colline = not possible

Examples:

• en haut du château because château is masculine
• en haut de la colline because colline is feminine

In this sentence, on is an indefinite subject. It often means something like:

• one
• people
• you
• we

In natural English, on voit la mer is often translated as:

• you can see the sea
• we can see the sea
• the sea is visible

The literal meaning of on is sometimes close to one, but in everyday French it is much more common and much more natural than English one.

Because on always takes the third-person singular verb form.

So:

• je vois
• tu vois
• il/elle/on voit
• nous voyons
• vous voyez
• ils/elles voient

Even when on means we in everyday speech, the verb still stays singular:

• On voit la mer = literally singular grammar, even if the meaning may feel plural in English

Because French usually needs an article before a noun in this kind of sentence.

So:

• la mer = the sea

French does not usually omit articles the way English sometimes can. A bare noun like mer would sound incomplete here.

Also, voir normally takes a direct object:

• voir la mer
• voir la maison
• voir les montagnes

French often puts a place expression first to set the scene.

So:

• En haut de la colline, on voit la mer.

This structure is very natural because it first tells you where the action happens, then tells you what is seen.

It is a bit like saying:

• At the top of the hill, you can see the sea.

The comma reflects that pause in writing.

Yes, that is possible, but the sentence given is often more natural if you want to emphasize the location first.

Compare:

• En haut de la colline, on voit la mer. → starts by setting the scene
• On voit la mer en haut de la colline. → also understandable, but the location comes later

Beginning with En haut de la colline feels especially good in descriptive writing or when introducing a landscape.

Because in French, the simple verb voir often expresses what English would naturally say with can see.

So:

• On voit la mer can naturally mean you can see the sea

French does not always need pouvoir here. If you said on peut voir la mer, that would put more emphasis on the ability or possibility of seeing it.

So:

• on voit la mer = the sea is visible / you can see the sea
• on peut voir la mer = one is able to see the sea

They are similar, but not identical.

• en haut de la colline = high up on the hill / at the top of the hill
  This is the expression used here.

• sur la colline = on the hill
  This is less precise. It could mean anywhere on the hill, not necessarily near the top.

• au sommet de la colline = at the summit/top of the hill
  This sounds more exact and a bit more formal or specific.

So en haut de la colline suggests a position near the top, without sounding as exact as au sommet.

Yes. Du haut de la colline is also possible, but it has a slightly different feel.

• en haut de la colline = located at the top/high up on the hill
• du haut de la colline = from the top of the hill

So du haut de emphasizes the viewpoint more strongly.

Compare:

• En haut de la colline, on voit la mer. = up on the hill, the sea is visible
• Du haut de la colline, on voit la mer. = from the top of the hill, one can see the sea

Both are good, but du haut de is a little more explicitly from that vantage point.

A rough pronunciation is:

ahn oh duh la ko-leen, on vwah la mehr

A few useful notes:

• en in en haut is nasal
• haut has a silent t
• colline sounds roughly like ko-leen
• voit sounds like vwah; the final t is silent
• mer sounds roughly like mehr, with a French r

A more IPA-style pronunciation is:

/ɑ̃ o də la kɔ.lin, ɔ̃ vwa la mɛʁ/

It is neutral and completely natural French.

Nothing in it is especially formal or especially casual. It would fit well in:

• a description of scenery
• a textbook
• travel writing
• ordinary speech

The use of on makes it feel very natural in modern French.