En el mercado he comprado medio kilo de tomates y un kilo de naranjas.

Spanish often puts place or time information at the beginning for context or emphasis. Starting with En el mercado means something like At the market / In the market.

It is not the only possible word order. You could also say:

• He comprado medio kilo de tomates y un kilo de naranjas en el mercado.

Both are correct. The version with En el mercado first sets the scene before saying what was bought.

In Spanish, you usually need an article with nouns like mercado when talking about a specific place in a normal, natural way.

So:

• en el mercado = at the market

Leaving out the article here would sound ungrammatical in standard Spanish.

He comprado is the present perfect:

• he = present tense of haber
• comprado = past participle of comprar

In Spain, the present perfect is very commonly used for actions in the recent past, especially when the time period is still connected to the present, such as today, this morning, or when the result matters now.

So he comprado can feel like:

• I have bought
• or in natural English, often just I bought, depending on context

In many parts of Latin America, people would more often say compré in this kind of sentence.

Not exactly.

There is overlap, but Spanish and English do not use the present perfect in exactly the same way.

In Spain, he comprado is often used for a recent completed action that still feels connected to the present. English sometimes uses I have bought, but very often natural English would simply say I bought.

So the grammar looks similar, but the usage is not always identical.

Because medio means half, so:

• medio kilo = half a kilo
• un kilo = one kilo

Spanish does not normally say un medio kilo here.

So the contrast is:

• medio kilo de tomates
• un kilo de naranjas

That is the standard way to express these quantities.

After a quantity expression, Spanish usually uses de before the thing being measured.

So:

• medio kilo de tomates
• un kilo de naranjas
• dos litros de leche
• una docena de huevos

This is very similar to English half a kilo of tomatoes and a kilo of oranges.

Because the sentence is talking about an amount made up of multiple items.

• medio kilo de tomates = a quantity containing several tomatoes
• un kilo de naranjas = a quantity containing several oranges

After a measure noun like kilo, Spanish normally uses the plural count noun if you mean multiple individual items.

After quantity expressions like medio kilo de or un kilo de, Spanish normally does not use an article.

So you say:

• medio kilo de tomates
• un kilo de naranjas

not:

• medio kilo de los tomates
• un kilo de las naranjas

You would only use los / las if you meant some specific tomatoes or oranges already known in the conversation, for example: el kilo de las naranjas que me recomendaste.

Because medio agrees with kilo, and kilo is masculine.

• medio kilo
• un kilo

If the noun being counted were feminine, then medio would change:

• media docena = half a dozen

So the form depends on the noun that follows medio, not on the fruit.

Yes. Kilo is a very common shortened form of kilogramo.

So:

• un kilo de naranjas
• un kilogramo de naranjas

Both are correct, but kilo is much more natural in everyday speech.

Yes. That is completely correct.

Spanish word order is fairly flexible. The difference is mainly about emphasis:

• En el mercado he comprado... puts the location first
• He comprado ... en el mercado puts the action first

Both mean the same basic thing.

Spanish normally uses y for and.

It changes to e only before words that begin with an i sound, such as:

• padres e hijos
• España e Italia

But un does not begin with an i sound, so it stays:

• tomates y un kilo de naranjas

So y is correct here.