No mercado, a balança mostra um quilo de tomate.

Because no is a contraction of em + o.

• em = in / at
• o = the
• em o mercado becomes no mercado

This kind of contraction is very common in Portuguese:

• em + a = na
• em + os = nos
• em + as = nas

So no mercado literally corresponds to in the market or at the market.

No mercado is a location phrase placed at the beginning of the sentence for context: At the market, ...

The comma helps separate that introductory element from the main clause. It is very natural in writing.

Without the comma, No mercado a balança mostra..., the sentence is still understandable, but the comma makes it clearer and smoother.

Here, balança means scale or weighing scale.

It does not mean the abstract idea of balance in this sentence. In a market context, a balança is the machine used to weigh produce, meat, etc.

Examples:

• A balança está quebrada. = The scale is broken.
• Coloque as frutas na balança. = Put the fruit on the scale.

Mostra is the third-person singular present form of mostrar = to show.

So:

• a balança = the scale
• mostra = shows

Together: the scale shows

The subject is singular, so the verb is singular too.

A quick conjugation of mostrar in the present:

• eu mostro
• você/ele/ela mostra
• nós mostramos
• vocês/eles/elas mostram

Yes. All three can work, with slightly different shades of meaning:

• mostra = shows
• marca = reads / registers / shows on a dial or display
• indica = indicates

In everyday Brazilian Portuguese, a balança marca um quilo is also very common.

So these are all natural:

• A balança mostra um quilo de tomate.
• A balança marca um quilo de tomate.
• A balança indica um quilo de tomate.

Because quilo is masculine.

It is a short form of quilograma, which is also masculine:

• o quilograma
• o quilo

So you say:

• um quilo
• dois quilos
• meio quilo

Not:

• uma quilo

Yes. Quilo is the common everyday short form of quilograma.

Both are correct, but quilo is much more natural in normal speech:

• um quilo de tomate = very common
• um quilograma de tomate = correct, but more formal or technical

This is similar to English using kilo instead of kilogram in casual situations.

In Brazilian Portuguese, after expressions of weight, quantity, or measure, a food item can sometimes appear in the singular when it is being treated more like a product or substance rather than as separate individual items.

So um quilo de tomate is natural in a market context.

You may also hear um quilo de tomates. The difference is subtle:

• de tomate = tomato as a product/commodity
• de tomates = individual tomatoes

In everyday Brazilian usage, especially with produce sold by weight, the singular is very common.

After quantity expressions like um quilo de..., Portuguese often uses the noun directly, without an article.

So:

• um quilo de tomate
• dois quilos de batata
• meio quilo de carne

Using an article there would usually sound unnatural:

• um quilo do tomate would suggest one kilo of the tomato or of that specific tomato, which is a different idea.

It can function as both, just like English a/an and one sometimes overlap.

In um quilo, it basically means one kilo.

With units of measurement, um often feels more numerical than just an indefinite article:

• um quilo
• um litro
• um metro

So although um is also the indefinite article a/an, here the meaning is strongly tied to the number one.

Yes, that is the idea.

The sentence means the reading on the scale is one kilo. Portuguese often expresses this exactly the same way English does:

• A balança mostra um quilo.
• The scale shows one kilo.

Then de tomate specifies what weighs that much.

Yes. In Brazilian Portuguese, mercado can refer to a market in a general sense, and in everyday speech it can also refer to a grocery-type place.

Depending on context, Brazilians may also say:

• supermercado = supermarket
• feira or feira livre = open-air street market

So No mercado is broad and natural.

The structure is:

• No mercado = location
• a balança = subject
• mostra = verb
• um quilo de tomate = object / what the scale shows

So, very literally:

• At the market,
• the scale
• shows
• one kilo of tomato

This word order is completely natural in Portuguese. The location phrase comes first for context, then the main statement follows.

Yes.

You could simply say:

• A balança mostra um quilo de tomate.

That still works perfectly. No mercado just sets the scene and tells you where this is happening.

So the first part is optional context, not something required by the grammar of the main sentence.