Se eu encontrar a meia preta, vou lavar as duas hoje.

Because Portuguese uses the future subjunctive after se when talking about a possible future situation.

So:

• Se eu encontrar... = If I find...

This is the normal pattern in Portuguese for future conditions:

• Se eu tiver tempo, vou. = If I have time, I’ll go.
• Se ele chegar cedo, jantamos juntos. = If he arrives early, we’ll have dinner together.

Using encontro here would sound wrong in standard Portuguese, and encontrarei would not fit after se in this kind of sentence.

For many regular verbs, the future subjunctive looks exactly like the personal infinitive or very similar to other forms, so it can be confusing.

With encontrar, the form for eu in the future subjunctive is:

• quando eu encontrar
• se eu encontrar

That same form happens to look like the dictionary form encontrar, but in this sentence it is functioning as a conjugated verb, not just an infinitive.

A good clue is the structure:

• se eu + verb
• quando eu + verb

After these words, Portuguese often uses the future subjunctive when referring to the future.

Yes, meia can mean different things depending on context.

In this sentence, a meia preta means:

• the black sock

But meia can also mean:

• half in some contexts
• the feminine form of meio/meia

Here, the article a and the context of as duas make it clear that meia means sock.

Because meia meaning sock is a feminine noun in Portuguese.

So you use feminine words with it:

• a meia
• a meia preta
• duas meias pretas

That is why the adjective also changes:

• preto → masculine
• preta → feminine

So a meia preta = the black sock

Because adjectives in Portuguese usually agree with the noun in gender and number.

Since meia is feminine singular, the adjective must also be feminine singular:

• a meia preta = the black sock

Compare:

• o sapato preto = the black shoe
• a meia preta = the black sock
• as meias pretas = the black socks

As duas means both of them or the two.

In this sentence, it refers to the two socks of the pair:

• Se eu encontrar a meia preta, vou lavar as duas hoje.
• If I find the black sock, I’m going to wash both today.

So the speaker has one sock already, is missing the black one, and says that if they find it, they will wash both socks today.

Because Portuguese often leaves out a noun when it is already clear from context.

So:

• vou lavar as duas
• literally: I’m going to wash the two
• natural meaning: I’m going to wash both (socks)

This is very common. Portuguese often avoids repeating the noun when everyone knows what is being referred to.

You could say vou lavar as duas meias, but it sounds more explicit and less natural in this context.

In Brazilian Portuguese, the construction ir + infinitive is very common for the future.

So:

• vou lavar = I’m going to wash / I will wash

This is often more common in everyday speech than the simple future:

• lavarei

Both are grammatically correct, but vou lavar sounds more natural and conversational in Brazil.

Yes. This is a very common and natural pattern for real future possibilities:

• Se + future subjunctive, main clause in future / near future / present with future meaning

Examples:

• Se eu encontrar a chave, vou sair.
• Se ela ligar, eu respondo.
• Se chover, vamos ficar em casa.

In your sentence:

• Se eu encontrar a meia preta = if I find the black sock
• vou lavar as duas hoje = I’ll wash both today

Yes, hoje can move around, but the version in the sentence sounds very natural.

• vou lavar as duas hoje = very natural
• hoje vou lavar as duas = also natural, with more focus on today
• vou hoje lavar as duas = possible, but less common in everyday speech

Portuguese word order is flexible, but some positions sound more natural than others depending on emphasis.

By itself, a meia preta just means the black sock.

In this sentence, the most likely meaning is that the speaker is looking for one specific sock, probably the missing one from a pair. Then as duas shows that there are two socks in total.

So the implied idea is something like:

• If I find the black sock that’s missing, I’ll wash both socks today.

Yes, you often could.

Both achar and encontrar can mean to find, but there can be slight differences:

• encontrar = to find, to come across, to locate
• achar = to find, but also to think

In many everyday situations, both work:

• Se eu encontrar a meia preta...
• Se eu achar a meia preta...

Encontrar may sound a little more clearly like locate/find, while achar is extremely common in speech.

Usually, no. Meia is singular: one sock.

• uma meia = one sock
• duas meias = two socks

If you want to talk about the pair, Portuguese usually still refers to two individual socks, not one unit in the same way English sometimes does informally.

That is why the sentence says:

• a meia preta = the black sock
• as duas = the two socks / both socks

Yes, eu could be omitted:

• Se encontrar a meia preta, vou lavar as duas hoje.

Portuguese often drops subject pronouns because the verb form already gives enough information.

However, in this sentence, eu may be included for clarity or emphasis. Since encontrar has a form that looks identical for different subjects in some contexts, keeping eu can make the sentence easier to understand.

So both are possible, but Se eu encontrar... is very clear and natural.