Antes de meus amigos chegarem, eu preciso varrer a sala e limpar a mesa.

Because Portuguese is using the personal infinitive here.

After antes de, if the subject of the infinitive is stated, Portuguese often changes the infinitive to match that subject. Since meus amigos is third person plural, the verb becomes chegarem.

• chegar = to arrive
• chegarem = for them to arrive

So Antes de meus amigos chegarem is the natural way to say before my friends arrive.

The personal infinitive is a special feature of Portuguese. It is an infinitive that can show who is doing the action.

For chegar, some forms are:

• chegar = to arrive
• chegarmos = for us to arrive
• chegarem = for them to arrive

English does not really have a direct equivalent. In this sentence, chegarem shows that the people arriving are my friends.

In standard Portuguese, chegarem is the correct form here, because meus amigos is an explicit plural subject.

So the standard version is:

• Antes de meus amigos chegarem

Using chegar instead would sound nonstandard in careful Portuguese.

Because the expression here is antes de.

Portuguese uses antes de before a noun or an infinitive structure:

• antes de sair = before leaving
• antes do almoço = before lunch
• antes de meus amigos chegarem = before my friends arrive

So the de is part of the normal grammar of the expression.

Both are grammatical, but meus amigos is the more neutral and common form in Brazilian Portuguese.

• meus amigos = my friends
• os meus amigos = also possible, with the definite article

Portuguese sometimes uses possessives with an article and sometimes without one. In Brazil, dropping the article is very common, especially in everyday speech.

Because amigos is a masculine plural noun.

Possessive words must agree with the noun they describe:

• meu amigo = my friend
• meus amigos = my friends
• minha amiga = my friend
• minhas amigas = my friends

If the group is all female, you would say minhas amigas. If it is a mixed group or just grammatically masculine, meus amigos is correct.

Yes, it can usually be left out.

Portuguese often omits subject pronouns because the verb form already shows the subject:

• eu preciso
• preciso

Both mean I need.

In this sentence, eu is optional. It may be included for emphasis, clarity, or natural rhythm.

Because they come after preciso.

The pattern is:

• precisar + infinitive

So:

• preciso varrer = I need to sweep
• preciso limpar = I need to clean

This is a very common structure in Portuguese. Unlike English, Portuguese does not need a separate word like to before the second verb in this pattern.

Because one preciso can apply to both verbs.

So:

• eu preciso varrer a sala e limpar a mesa

means:

• I need to sweep the room and clean the table

Portuguese, like English, often avoids repeating the same verb when it clearly applies to both actions.

Because Portuguese uses definite articles much more often than English.

So these are the natural expressions:

• varrer a sala
• limpar a mesa

Even when English might say just sweep the room or clean the table, Portuguese usually includes the article.

Here it is a verb.

It comes from precisar, which means to need:

• eu preciso = I need

There is also an adjective preciso meaning precise or accurate, but that is not what is happening in this sentence.

Yes. That would be very natural in Brazilian Portuguese.

• Eu preciso varrer a sala. = I need to sweep the room.
• Eu tenho que varrer a sala. = I have to sweep the room.

Both are common. Very roughly:

• preciso = need to
• tenho que = have to

In everyday speech, both work well in sentences like this.

Yes, but the grammar changes.

You have two common options:

• antes de meus amigos chegarem
• antes que meus amigos cheguem

The first uses an infinitive structure, specifically the personal infinitive.
The second uses antes que + subjunctive.

Both are correct and natural. The version in your sentence is especially useful because it shows a very common Portuguese pattern.

Yes.

The original sentence starts with the time clause:

• Antes de meus amigos chegarem, eu preciso varrer a sala e limpar a mesa.

You could also say:

• Eu preciso varrer a sala e limpar a mesa antes de meus amigos chegarem.

Both are correct. The difference is mainly one of emphasis and flow, not basic meaning.