Hoje vou comprar courgette no mercado.

It is understandable, but it sounds a little less natural than a native-speaker version.

In Portugal, you would more often hear:

• Hoje vou comprar uma curgete no mercado. = one courgette
• Hoje vou comprar curgetes no mercado. = courgettes in general / several courgettes

Two things make the original sentence feel slightly unusual:

• curgete is the standard European Portuguese spelling
• the bare singular courgette without uma is not the most natural choice in a normal sentence

So the structure is fine, but the noun phrase is the part most likely to be adjusted.

Because Portuguese often drops subject pronouns when the verb already makes the subject clear.

• vou already means I go / I am going
• so Hoje vou comprar... naturally means Today I’m going to buy...

You can still say Eu hoje vou comprar..., but adding eu usually gives extra emphasis, for example:

• Eu hoje vou comprar curgetes, não amanhã.
  = I’m buying courgettes today, not tomorrow.

This is very common in Portuguese and is one of the first things English speakers notice.

Vou comprar is the very common ir + infinitive future structure:

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

Together, vou comprar means I’m going to buy.

This is the normal everyday way to talk about future plans in Portuguese. It does not necessarily mean physical movement. It just expresses intention or a planned future action.

A more formal or literary future would be:

• comprarei = I will buy

So:

• Hoje vou comprar curgetes no mercado. = everyday, natural
• Hoje comprarei curgetes no mercado. = correct, but more formal or written

Putting Hoje at the beginning is very natural because it sets the time frame right away.

Portuguese often places time expressions first:

• Hoje vou comprar curgetes no mercado.
• Amanhã vou trabalhar.
• Ontem fui ao supermercado.

It is also possible to move hoje later:

• Vou comprar curgetes no mercado hoje.

That is grammatical too, but starting with Hoje is often the most natural way to introduce the sentence.

Usually, no.

In a short, simple sentence like this, Portuguese normally does not put a comma after a fronted time word:

• Hoje vou comprar curgetes no mercado.

A comma could appear in special cases for emphasis or rhythm, but it is not the neutral default here.

So for learners, the safest choice is:

• Hoje vou comprar...
  not
• Hoje, vou comprar...

No is a contraction:

• em + o = no

So:

• no mercado = in the market / at the market

This happens very often in Portuguese:

• no = em + o
• na = em + a
• nos = em + os
• nas = em + as

So:

• o mercado = the market
• no mercado = in/at the market

This sentence uses no mercado because it describes where the buying happens.

A useful contrast is:

• Vou ao mercado. = I’m going to the market.
  focus: destination
• Vou comprar curgetes no mercado. = I’m going to buy courgettes at the market.
  focus: place where the buying happens

That is one of the reasons the sentence feels a bit unusual.

In Portuguese, a singular countable noun usually needs a determiner such as um/uma or o/a.

So if you mean one courgette, you would normally say:

• vou comprar uma curgete

If you mean courgettes in general or several of them, you would normally say:

• vou comprar curgetes

The bare singular form can sometimes appear in list-like or note-like language, such as a shopping list, but in a full sentence it sounds less natural.

So for most learner situations, these are the better choices:

• Hoje vou comprar uma curgete no mercado.
• Hoje vou comprar curgetes no mercado.

In European Portuguese, the standard spelling is usually curgete.

So if you are learning Portuguese from Portugal, curgete is the form to learn first.

You may still see courgette sometimes, because the word originally came from French and spelling can vary in informal contexts, labels, or imported usage. But for standard European Portuguese:

• curgete = preferred spelling in Portugal
• curgetes = plural

Also, this is different from Brazilian Portuguese, where the usual word is:

• abobrinha

So:

• Portugal: curgete
• Brazil: abobrinha