Na mercearia, o arroz é mais barato do que no supermercado.

Because Portuguese combines the preposition em with the definite article:

• em + a = na
• em + o = no

So:

• mercearia is feminine, so em a mercearia becomes na mercearia
• supermercado is masculine, so em o supermercado becomes no supermercado

In this sentence, na and no mean something like in the / at the.

Portuguese often uses the definite article with nouns when talking about something in a general or familiar way, especially when the noun is the subject of the sentence.

So o arroz é mais barato... sounds natural in Portuguese.

Compare:

• Comprei arroz = I bought rice
• O arroz é caro = Rice is expensive

English often leaves the article out in these general statements, but Portuguese very often keeps it.

Because the adjective must agree with the noun it describes.

Here, arroz is:

• masculine
• singular

So the adjective is also masculine singular:

• o arroz é barato

If the noun were feminine, you would use barata:

• A massa é mais barata

If it were plural:

• Os legumes são mais baratos
• As maçãs são mais baratas

Here é is used because the sentence sounds like a general fact or usual comparison: rice is cheaper in one place than in another.

• é mais barato = it is cheaper, as a general characteristic
• está mais barato = it is cheaper right now, possibly temporarily

So:

• O arroz é mais barato na mercearia suggests a general price difference
• O arroz está mais barato na mercearia could suggest a temporary special price or current situation

Both are possible in the right context, but é is very natural for a general comparison.

This is the normal comparative pattern for more ... than:

• mais + adjective + do que

So:

• mais barato do que = cheaper than
• literally, more cheap than

Other examples:

• mais caro do que = more expensive than
• mais rápido do que = faster than
• mais difícil do que = more difficult than

This is a very useful pattern to memorize.

Yes, in many cases you will hear both que and do que after comparatives.

So both of these are possible:

• mais barato do que no supermercado
• mais barato que no supermercado

For a learner, do que is a very safe and clear choice, and it is extremely common.

Because Portuguese, like English, often leaves out repeated words when they are already clear from the context.

The full idea is something like:

• O arroz é mais barato do que é no supermercado

But repeating é and the rest would sound unnecessary. So Portuguese naturally shortens it.

This kind of omission is very common.

Putting Na mercearia first gives special emphasis to the place. It is like setting the scene first:

• Na mercearia, o arroz é mais barato do que no supermercado.

This word order is perfectly natural.

You could also say:

• O arroz é mais barato na mercearia do que no supermercado.

That version is also correct. The main difference is emphasis:

• Na mercearia, ... highlights the location first
• O arroz ... starts with the thing being discussed

The comma separates the introductory location phrase from the rest of the sentence.

It helps show that Na mercearia is being used as a frame or topic for what follows.

In a short sentence like this, the comma is often a style choice rather than an absolute necessity, but it is very natural here because the speaker is fronting the location for emphasis.

In European Portuguese, mercearia usually means a small grocery shop or neighbourhood grocer, not a large supermarket.

So the contrast here is:

• mercearia = small local grocery shop
• supermercado = supermarket

That cultural nuance matters, because the sentence compares prices between a smaller local shop and a bigger supermarket.

In European Portuguese, unstressed vowels are often reduced, so the sentence may sound more compressed than an English speaker expects.

A rough guide is:

• Na mercearia ≈ nuh mər-suh-ree-UH
• o arroz ≈ u uh-RROSH
• é mais barato ≈ eh maizh buh-RAH-too
• do que no supermercado ≈ du kuh nu soo-per-mer-KAH-du

A few important points:

• o often sounds like a very short u
• the s in mais barato sounds like zh because of linking
• the r in arroz is a strong Portuguese r
• many unstressed vowels are weaker than in English

So in real speech, the whole sentence may sound faster and more reduced than its spelling suggests.