A farofa e o vinagrete estão prontos.

Because Portuguese nouns have grammatical gender.

• farofa is feminine, so it takes a
• vinagrete is masculine, so it takes o

These are the singular definite articles, equivalent to the in English:

• a farofa = the farofa
• o vinagrete = the vinaigrette / the tomato-onion salad

Unlike English, Portuguese usually makes you show gender in the article.

Because the subject is plural.

Even though a farofa and o vinagrete are each singular by themselves, they are joined by e = and, so together they form a plural subject:

• A farofa
  • o vinagrete = they

So the verb must also be plural:

• está = is
• estão = are

That is why the sentence says estão prontos = are ready.

Because estar pronto normally describes a state or condition: something is ready, done, or prepared at this moment.

In Portuguese, ser and estar both can mean to be, but they are used differently:

• ser = identity, essence, general characteristic
• estar = condition, state, location, temporary situation

So:

• A farofa e o vinagrete estão prontos = The farofa and the vinaigrette are ready.

Using ser here would sound wrong in normal Portuguese.

Because the adjective agrees with the whole group, not just with farofa.

The group is:

• a farofa = feminine
• o vinagrete = masculine

When a group contains at least one masculine noun, standard Portuguese normally uses the masculine plural form for the adjective.

So:

• singular masculine: pronto
• singular feminine: pronta
• plural masculine (or mixed group): prontos
• plural feminine: prontas

That is why the sentence ends with prontos.

Yes.

For example:

• A farofa e a salada estão prontas.

Both nouns there are feminine, so the adjective becomes prontas.

This is basic adjective agreement in Portuguese:

• number must match: singular/plural
• gender must match: masculine/feminine

Here, prontos means ready.

With food, it often means:

• finished
• prepared
• ready to serve
• done

So in this sentence, it means the dishes are prepared and can be eaten or served.

Usually, in a sentence like this, the articles sound natural and are normally kept:

• A farofa e o vinagrete estão prontos.

If you remove them:

• Farofa e vinagrete estão prontos.

it can still be understandable, but it sounds less natural in many everyday contexts, especially if you are talking about the specific dishes that were prepared.

Portuguese often uses definite articles more than English does.

Yes. Noun gender in Portuguese is not determined only by the last letter.

Many learners first notice:

• words ending in -a are often feminine
• words ending in -o are often masculine

But words ending in -e can be either masculine or feminine. You usually have to learn the gender with the noun:

• o vinagrete
• o leite
• a carne
• a noite

So it is best to memorize nouns together with their article.

A simple approximation is es-TOWN, but with a nasal ending.

A few helpful points:

• the stress is on the last syllable: -tão
• ão is a nasal sound that does not exist exactly in English
• the es at the beginning is usually light

A rough learner-friendly version:

• estão ≈ ehs-TOWN with the end pronounced through the nose

If you want to sound more natural, pay special attention to the nasal ão.

A simple Brazilian Portuguese approximation:

• farofa = fa-RO-fa
• vinagrete = vee-na-gre-TE

Stress:

• farofa → stress on RO
• vinagrete → stress on TE

Also:

• Portuguese r can sound different from English r
• unstressed vowels are often lighter than English learners expect

Yes, absolutely.

• A farofa e o vinagrete já estão prontos. = The farofa and the vinaigrette are already ready.

Adding já gives the idea of already. This is very common in everyday speech.

In careful or standard speech, yes: estão.

In casual speech, many Brazilians reduce it:

• tão prontos

So you may hear:

• A farofa e o vinagrete tão prontos.

This is common in spoken Brazilian Portuguese, but for learning and writing, estão prontos is the best form to use.

It refers to two separate things:

• a farofa
• o vinagrete

They are linked by e = and, so the meaning is that both dishes are ready.

That is also why the verb and adjective are plural:

• estão
• prontos

Yes. That is grammatically correct.

• A farofa e o vinagrete estão prontos.
• O vinagrete e a farofa estão prontos.

Both mean the same thing. The difference is just emphasis or the order in which the speaker chooses to mention the dishes.

The agreement stays the same:

• plural verb: estão
• masculine plural adjective for the mixed group: prontos