Na mercearia, o milho estava mais barato do que as azeitonas, por isso trouxe os dois.

Na is a contraction of em + a.

• em = in / at
• a = the, feminine singular

So na mercearia means in the grocer's shop or at the grocery shop.

Mercearia usually means a grocery shop or small grocer's. In Portugal, it often suggests a smaller local food shop, not necessarily a big supermarket.

So it is a bit more specific than just shop, and often smaller than a supermercado.

Portuguese uses definite articles much more often than English.

So where English might say corn and olives, Portuguese often says o milho and as azeitonas.

Here, the articles make the nouns sound like the specific items being talked about in that situation, such as the corn and olives available in the shop.

Milho is often used as a mass noun, like corn in English.

So o milho does not necessarily mean one single piece of corn. It can mean corn as a food product in general. That is why the singular is natural here.

Estava is the imperfect form of estar. It is used here to describe a situation or state in the background:

• o milho estava mais barato = the corn was cheaper

This sounds natural because the sentence is describing how things were at that moment in the shop.

• foi would sound more like a completed event
• era would sound less natural here, because prices are usually treated as a temporary state rather than an essential characteristic

This is the standard comparative structure:

• mais
  • adjective + do que

So:

• mais barato = cheaper / more cheap
• mais barato do que as azeitonas = cheaper than the olives

In English, we usually say cheaper than. In Portuguese, the literal structure is closer to more cheap than.

Because barato agrees with o milho, which is masculine singular.

In this part of the sentence, the corn is the thing being described:

• o milho estava mais barato

The olives are only the comparison point:

• do que as azeitonas

So the adjective matches milho, not azeitonas.

Yes. Both are used.

• mais barato do que
• mais barato que

Both mean the same thing. Do que is very standard and often taught as the safer full form, so it is a good choice for learners.

Por isso means therefore, so, or for that reason.

It introduces a result:

• the corn was cheaper than the olives,
• por isso I brought both.

It is a very common way to link cause and result in Portuguese.

Because por isso introduces the consequence of what came before.

The comma helps separate:

• the situation: o milho estava mais barato do que as azeitonas
• the result: por isso trouxe os dois

It works a lot like a pause before so or therefore in English.

Trouxe is the preterite of trazer, meaning to bring.

Here it means I brought.

It is irregular:

• infinitive: trazer
• preterite: trouxe

So trouxe os dois means I brought both.

Portuguese usually distinguishes between:

• trazer = to bring, toward the speaker or destination
• levar = to take, away from the speaker or starting point

If the speaker is talking from the point of view of where the items ended up, trouxe is natural: I brought both.

English often uses bring and take less strictly in everyday speech, so this can feel unfamiliar to learners.

Os dois means both or the two.

It refers to the two things just mentioned:

• o milho
• as azeitonas

So trouxe os dois means I brought both.

Because the two things referred to are mixed in grammatical gender:

• o milho = masculine
• as azeitonas = feminine

When Portuguese refers to a mixed group, the default plural is usually masculine. So:

• os dois = the two / both

That is why os dois is correct here.

Yes, you could say trouxe ambos.

It is grammatical, but os dois sounds more natural and everyday in this sentence. Ambos is a bit more formal or written in tone.