Se o pepino estiver fresco, eu também vou comprar repolho para a salada.

Because this sentence is talking about a possible future condition.

After se meaning if, Portuguese often uses the future subjunctive when the condition is about the future:

• Se o pepino estiver fresco... = If the cucumber is fresh / if the cucumber turns out to be fresh...

Using está would sound wrong in this kind of future-looking sentence.

A useful contrast:

• Se o pepino estiver fresco, eu vou comprar repolho.
  Future possibility

• Se o pepino está fresco, eu compro.
  More like a general or repeated situation, and not the best match for this context

Estiver is the future subjunctive form of the verb estar.

So:

• infinitive: estar
• future subjunctive, eu/ele/você form: estiver

This form is very common after words like:

• se = if
• quando = when
• assim que = as soon as
• logo que = once/as soon as

Examples:

• Se ele estiver em casa...
• Quando eu tiver tempo...
• Assim que você chegar...

English usually does not mark this with a special verb form, which is why it can feel unfamiliar.

Because fresco here describes a temporary condition of the cucumber: whether it is fresh right now.

Portuguese usually uses:

• estar for states or conditions
• ser for more permanent or defining characteristics

So for food freshness, estar fresco is the natural choice.

• O pepino está fresco. = The cucumber is fresh.

Using ser fresco would not normally mean that the food is fresh in the usual sense.

Vou comprar is the very common near future / periphrastic future in Portuguese:

• vou comprar = I am going to buy / I will buy

It is extremely natural in everyday Brazilian Portuguese.

You could also say:

• comprarei repolho

But that sounds more formal, more written, or less conversational.

So:

• eu vou comprar repolho = normal spoken Brazilian Portuguese
• eu comprarei repolho = correct, but more formal

Também means also or too.

In this sentence, it shows that buying cabbage is an additional action. The speaker is not only doing something else already understood from the context, but also buying cabbage.

So the sense is:

• I will also buy cabbage for the salad

The exact thing being added depends on the larger context. For example, maybe the speaker is already buying tomatoes or lettuce, and cabbage is one more item.

This is a very common question.

o pepino

The article o can make pepino sound like:

• a specific cucumber already being discussed
• the cucumber being checked at the store
• or sometimes a generic ingredient understood from context

repolho without an article

After comprar, Portuguese often allows a bare noun when talking about buying some amount of a food item in a general sense:

• comprar repolho = buy cabbage / some cabbage

This sounds very natural.

If you said:

• comprar o repolho

that would usually mean a specific cabbage already identified.

So the difference is roughly:

• vou comprar repolho = I will buy cabbage
• vou comprar o repolho = I will buy the cabbage

Because fresco must agree with pepino, and pepino is masculine singular.

So:

• o pepino fresco
• o pepino estiver fresco

If the noun changed, the adjective would change too:

• a alface está fresca = the lettuce is fresh
• os pepinos estão frescos = the cucumbers are fresh
• as verduras estão frescas = the vegetables are fresh

So fresco matches pepino.

It means for the salad.

Here:

• para = for
• a salada = the salad

So para a salada means the cabbage is being bought with the purpose of using it in the salad.

In everyday Brazilian Portuguese, people very often say:

• pra salada

That is the colloquial contraction of para a salada.

So these are both natural:

• para a salada = a bit more neutral/full
• pra salada = very common in speech

Yes. Portuguese often drops subject pronouns when the meaning is clear from the verb form.

So you could say:

• Se o pepino estiver fresco, também vou comprar repolho para a salada.

This sounds perfectly natural.

Including eu can add:

• emphasis
• contrast
• clarity

For example, eu também can emphasize that I too will do this.

So both are correct:

• eu também vou comprar...
• também vou comprar...

Yes, but the emphasis can shift a little.

Your sentence:

• eu também vou comprar repolho para a salada

This usually emphasizes that I also will buy it.

Other possibilities:

• eu vou também comprar repolho para a salada
  Grammatically possible, but less natural in everyday speech

• eu vou comprar repolho também para a salada
  Possible, but the placement sounds less neutral

• eu vou comprar repolho para a salada também
  This can sound like for the salad too, depending on context

So the original placement is very natural and probably the best choice for most learners.

This word can be tricky because of lh.

Approximate Brazilian pronunciation

repolho ≈ heh-PO-lyoo or reh-PO-lyoo, depending on the accent

A few notes:

• r at the beginning of a word in Brazilian Portuguese is often pronounced like an English h or a stronger throat sound
• lh is like the lli sound in million for many English speakers, though not exactly the same
• the stress is on po

So:

• repolho → stress on the middle syllable

If you want a very rough guide:

• re-PO-lho

The most important part for learners is not to pronounce lh like a simple English l.