La zucca arancione è sul ripiano vicino alle patate.

Because zucca is a feminine singular noun in Italian. The definite article for a feminine singular noun is usually la.

So:

• la zucca = the pumpkin
• la casa = the house
• la patata = the potato

If the noun were masculine singular, you would normally use il instead.

In Italian, descriptive adjectives often come after the noun, much more often than in English.

So Italian commonly says:

• la zucca arancione
• la casa grande
• il libro interessante

rather than putting the adjective first.

Putting an adjective before the noun is sometimes possible in Italian, but it often changes the tone, emphasis, or sounds less natural in a basic descriptive sentence. Here, la zucca arancione is the normal choice.

Because arancione does not change for masculine vs. feminine in the singular.

So you get:

• il libro arancione
• la zucca arancione

But it does change in the plural:

• i libri arancioni
• le zucche arancioni

So in this sentence, zucca is feminine singular, and arancione stays arancione.

È is the third-person singular form of essere (to be), so here it means is.

The accent matters because:

• è = is
• e = and

So the accent helps distinguish the verb from the conjunction.

Examples:

• La zucca è sul ripiano. = The pumpkin is on the shelf.
• La zucca e le patate = The pumpkin and the potatoes

Sul is a contraction of:

• su = on
• il = the

So:

• su + il = sul

That is why sul ripiano means on the shelf / on the ledge / on the surface depending on context.

Other similar contractions are:

• su + la = sulla
• su + lo = sullo
• su + i = sui
• su + le = sulle

Because Italian usually uses the definite article where English often does too, and sometimes even where English might omit it.

Here, ripiano is a specific shelf/surface, so Italian uses the:

• sul ripiano = on the shelf

Saying su ripiano would sound incomplete or ungrammatical in standard Italian.

Also, notice that the article is hidden inside sul:

• sul = su + il

Because vicino is commonly used with the preposition a when followed by a noun.

So the full structure is:

• vicino a le patate

But a + le contracts to alle, giving:

• vicino alle patate

This is very common:

• vicino alla porta = near the door
• vicino al tavolo = near the table
• vicino ai libri = near the books
• vicino alle patate = near the potatoes

Because patate is feminine plural.

The singular is:

• la patata

The plural is:

• le patate

After vicino a, the a combines with le:

• a + le = alle

So:

• vicino alle patate = near the potatoes

Italian word order is fairly flexible, although the sentence you have is a very natural neutral order.

This version is straightforward:

• La zucca arancione è sul ripiano vicino alle patate.

But Italian could also say:

• Sul ripiano vicino alle patate c’è la zucca arancione.

That version puts the location first and sounds more like On the shelf near the potatoes, there is the orange pumpkin.

So the original sentence is not the only possible order, but it is a normal and clear one.

Because sul means on the surface of, while nel means in / inside the.

So:

• sul ripiano = on the shelf/surface
• nel ripiano would suggest something more like inside a compartment, which is not the idea here

If the pumpkin is resting on top of the shelf, sul is the right choice.

Yes, often you could.

• vicino alle patate = near the potatoes
• accanto alle patate = next to / beside the potatoes

Vicino is a bit broader and simply means near.
Accanto a often suggests being more directly beside something.

So both can work, but vicino is slightly more general.