La vetta è sopra la valle.

Because Italian usually uses definite articles more often than English.

• la vetta = the summit
• la valle = the valley

Both vetta and valle are singular feminine nouns, so they take la.

Not exactly.

• vetta means summit, peak, or top
• montagna means mountain

So la vetta is the highest point of a mountain, not the whole mountain.

È is the third-person singular form of essere (to be), meaning is.

The accent is important because it distinguishes è (is) from e (and).

So:

• è = is
• e = and

Here, sopra means above or over.

So La vetta è sopra la valle means the summit is higher than the valley.

Compared with su:

• sopra usually emphasizes position above
• su often means on or on top of, though it can sometimes overlap with sopra

For example:

• Il libro è sul tavolo = The book is on the table
• Le nuvole sono sopra la città = The clouds are above the city

In your sentence, sopra is the natural choice because a summit is physically higher than a valley.

This is the normal Italian word order:

• La vetta = subject
• è = verb
• sopra la valle = prepositional phrase

So it follows the common pattern Subject + Verb + Location.

You could change the order for emphasis, but the basic sentence is most naturally:

La vetta è sopra la valle.

Many feminine nouns do end in -a, but not all of them.

Valle is a feminine noun even though it ends in -e:

• singular: la valle
• plural: le valli

So in Italian, noun gender is not determined only by the final letter. You often need to learn the article together with the noun.

A simple pronunciation guide:

• vetta ≈ VET-ta
• valle ≈ VAL-le
• è ≈ like eh
• sopra ≈ SO-pra

A few details:

• tt in vetta is pronounced more strongly than a single t
• ll in valle is like a normal Italian l, not like the English sound in million
• Stress:
  • VET-ta
  • VAL-le
  • SO-pra

Usually no, not with the same meaning.

sulla means on the, from su + la. That would suggest the summit is on the valley, which does not make sense in normal geography.

• sopra la valle = above the valley
• sulla valle = on the valley

So sopra la valle is the correct expression here.

It can do both.

In your sentence, it is followed by a noun phrase:

• sopra la valle = above the valley

But it can also stand alone when the place is understood:

• Il paese è sotto, il castello è sopra. = The town is below, the castle is above.

So sopra can be either:

• a preposition-like word before a noun phrase, or
• an adverb on its own

Yes, they could.

si trova literally means is found but is often used to mean is located.

So:

• La vetta è sopra la valle = The summit is above the valley
• La vetta si trova sopra la valle = The summit is located above the valley

The first is simpler and more direct. The second sounds a bit more descriptive or formal.

It would become:

Le vette sono sopra la valle.
= The summits are above the valley.

Changes:

• la vetta → le vette
• è → sono

If both nouns were plural:

Le vette sono sopra le valli.
= The summits are above the valleys.