Il rilevatore di fumo è sopra la porta.

Il means the.

Italian has different forms of the masculine singular definite article:

• il
• lo
• l’

You use il before most masculine singular nouns, including rilevatore.

You use lo before masculine singular nouns that begin with:

• s + consonant (like lo studente)
• z (like lo zaino)
• ps, gn, x, and a few others

Since rilevatore starts with a normal r, the correct article is il.

Yes. Rilevatore means detector.

It comes from the verb rilevare, which can mean to detect, to register, or to pick up depending on context.

So:

• rilevatore di fumo = smoke detector
• rilevatore di movimento = motion detector

In everyday Italian, rilevatore di fumo is a normal and natural way to say smoke detector.

Italian often uses the structure noun + di + noun where English uses a noun as an adjective.

So instead of literally saying something like smoke detector with smoke directly modifying detector, Italian says:

• rilevatore di fumo
• literally: detector of smoke

This pattern is very common in Italian:

• bicchiere di vino = glass of wine
• scarpe da tennis = tennis shoes
• sala da pranzo = dining room

So di fumo is the normal way to express for smoke / of smoke here.

È with an accent means is.

The accent is important because it distinguishes è from e:

• è = is
• e = and

Examples:

• La porta è aperta. = The door is open.
• La porta e la finestra = The door and the window

So the accent is not optional here.

Sopra means above or on top of / over, depending on context.

In this sentence, sopra la porta means above the door.

It is similar to su, but not always identical:

• su often means on
• sopra emphasizes above or over

For example:

• Il libro è sul tavolo. = The book is on the table.
• La lampada è sopra il tavolo. = The lamp is above the table / over the table.

With a smoke detector mounted high on a wall, sopra la porta is very natural because the idea is above the door, not necessarily directly resting on it.

Yes. Porta is a feminine singular noun, so it takes the feminine singular definite article la.

That gives:

• la porta = the door

Other forms would be:

• una porta = a door
• le porte = the doors

Even though many nouns ending in -a are feminine, that is a pattern, not an absolute rule. But porta is definitely feminine.

The word order is very straightforward:

Il rilevatore di fumo + è + sopra la porta

That is:

• subject = Il rilevatore di fumo
• verb = è
• location phrase = sopra la porta

This matches normal English order quite closely:

• The smoke detector
  • is
    • above the door

So this is a very beginner-friendly Italian sentence structurally.

You could, but it usually means something slightly different.

• sopra la porta = above the door
• sulla porta = on the door

So:

• If the detector is mounted on the wall above the door, sopra la porta is better.
• If something is physically attached to the door itself, sulla porta would make more sense.

That difference is important.

In a normal sentence like this, yes, Italian usually includes the definite article.

English often allows article use a bit more flexibly, but Italian commonly uses articles with nouns in ordinary statements:

• Il rilevatore di fumo è sopra la porta.

Leaving out the articles would sound unnatural in this context.

The plural would be:

I rilevatori di fumo sono sopra la porta.

Changes:

• il → i
• rilevatore → rilevatori
• è → sono

Di fumo does not change here, because fumo is still just the noun smoke in that expression.

If the doors were plural too, then you would say:

• I rilevatori di fumo sono sopra le porte.

A simple English-friendly guide is:

eel ree-leh-vah-TOH-reh dee FOO-moh eh SOH-prah lah POR-tah

A few helpful notes:

• r in Italian is usually rolled or tapped
• e in è is pronounced like eh
• di is dee
• gli is not in this sentence, so pronunciation is fairly simple

The main stress falls roughly here:

• rilevaTOre
• FUmo
• SOpra
• PORta

It sounds natural.

It is simple and clear, but it is also perfectly normal Italian. A native speaker would understand it immediately, and it does not sound awkward.

If anything, in real life someone might also say:

• C’è un rilevatore di fumo sopra la porta. = There’s a smoke detector above the door.

But your sentence is completely correct and natural too.