Il contrario di “aperto” è “chiuso”.

It means The opposite of ... is ....

So:

• Il contrario = the opposite
• di = of
• è = is

You can reuse this pattern with many words:

• Il contrario di caldo è freddo. = The opposite of hot is cold.
• Il contrario di grande è piccolo. = The opposite of big is small.

Il is the masculine singular definite article, meaning the.

In Il contrario di aperto è chiuso, contrario is a noun meaning opposite, so Italian normally uses the article: il contrario = the opposite.

You usually would not say just Contrario di aperto... in normal Italian. The article sounds natural and correct here.

Because contrario di is the standard way to say the opposite of.

So:

• il contrario di aperto = the opposite of open
• il contrario di vero = the opposite of true

In English, we use of. In Italian, that role is played by di here.

They are being used as dictionary-style adjective forms, or as general word labels.

In Italian, the basic citation form of many adjectives is the masculine singular, so:

• aperto = open
• chiuso = closed

If you were describing a feminine noun, you would change them:

• La porta è aperta. = The door is open.
• La porta è chiusa. = The door is closed.

If you were talking about masculine plural or feminine plural nouns, the forms would also change:

• I negozi sono aperti. = The shops are open.
• Le finestre sono chiuse. = The windows are closed.

Yes, but only if the thing you are referring to is feminine singular.

Examples:

• Il contrario di porta aperta è porta chiusa would sound unusual as a vocabulary statement, but grammatically aperta/chiusa agree with porta because porta is feminine.
• La finestra è aperta.
• La finestra è chiusa.

In your sentence, though, the speaker is talking about the words themselves in a general way, so aperto and chiuso are given in the masculine singular base form.

Because è is the verb is from essere.

The accent is important. It distinguishes è from e:

• è = is
• e = and

So:

• Il contrario di aperto è chiuso. = The opposite of open is closed.
• pane e burro = bread and butter

This accent should always be written.

A simple pronunciation guide is:

eel kon-TRAH-ryo dee ah-PAIR-toh eh kee-OO-zoh

A few notes:

• il sounds roughly like eel
• contrario has the stress on tra
• aperto has the stress on per
• chiuso begins with chiu-, where chi sounds like kee
• è is pronounced like eh

A more IPA-like version is:

[il konˈtraːrjo di aˈpɛrto ɛ ˈkjuːzo]

They can be both, depending on context.

As adjectives:

• una porta aperta = an open door
• un negozio chiuso = a closed shop

As past participles, they connect to verbs:

• aprire → aperto = opened
• chiudere → chiuso = closed

In your sentence, they are functioning like adjectives or vocabulary items: open and closed.

Yes, that is possible.

• il contrario di = the opposite of
• l'opposto di = the opposite of

Both are understandable and natural. In many contexts, il contrario di is very common when talking about vocabulary pairs and opposites.

So both of these work:

• Il contrario di aperto è chiuso.
• L'opposto di aperto è chiuso.

Not always.

Quotation marks are often used when talking about words as words, especially in teaching materials:

• Il contrario di “aperto” è “chiuso”.

But in many ordinary contexts, people may simply write:

• Il contrario di aperto è chiuso.

Both are understandable. The quotation marks just make it extra clear that you are discussing the words themselves, not describing a specific object.

This order is the most straightforward and natural:

• Il contrario di aperto è chiuso.

Italian does allow some flexibility, but not every alternative sounds equally natural.

For example, you could sometimes emphasize the result:

• Chiuso è il contrario di aperto.

This is grammatical, but it is more marked and less neutral. For a learner, the safest pattern is:

• Il contrario di X è Y.

It is most commonly used with adjectives when teaching opposites, but it can be used more broadly.

With adjectives:

• Il contrario di alto è basso. = The opposite of tall is short.
• Il contrario di facile è difficile. = The opposite of easy is difficult.

With adverbs:

• Il contrario di bene è male. = The opposite of well is badly / good is evil, depending on context.

With nouns, it is possible, though sometimes opposto or a different phrasing may sound better depending on meaning.

So yes, the pattern is flexible, but it is especially common for adjective pairs like aperto/chiuso.

Because contrario takes il, not lo.

Italian uses:

• il before many masculine singular nouns
• lo before masculine singular nouns beginning with certain sound combinations, such as z, s + consonant, gn, ps, and a few others

Examples:

• il contrario
• il libro
• lo studente
• lo zaino

Since contrario begins with a simple c sound, the correct article is il.