A caixa de transporte está pronta.

Because caixa meaning box / case / crate is a feminine noun in Portuguese, so it takes a in the singular: a caixa.

Be careful: caixa can also mean cashier / clerk in some contexts, and then you may hear o caixa or a caixa referring to a person. But in this sentence, it clearly means a box/carrier, so a caixa is correct.

Literally, it is something like box of transport or transport box, but natural English usually says carrier, transport box, shipping crate, or pet carrier, depending on context.

Portuguese often uses the pattern noun + de + noun where English prefers a compound noun or an adjective. So:

• caixa de transporte = transport box / carrier
• garrafa de água = water bottle
• sapato de corrida = running shoe

Because de transporte describes the type or purpose of the box: it is a box for transport.

If you said do transporte, that would usually mean of the transport or belonging to the transport, referring to some specific transport already known in the conversation. That is not the idea here.

So:

• caixa de transporte = a transport box / carrier
• caixa do transporte = the box of the transport system/vehicle/company/contextually specific transport

Because pronta describes a current state or condition: the carrier is ready now.

In Portuguese, estar is commonly used for temporary states, conditions, and situations:

• A caixa está pronta. = The box is ready.
• A porta está aberta. = The door is open.
• Estou cansado. = I am tired.

Using ser here would sound unnatural in normal Portuguese, because ready is not seen as a permanent characteristic of the box.

Because adjectives in Portuguese usually agree with the noun in gender and number.

• caixa is feminine singular
• so the adjective must also be feminine singular: pronta

Compare:

• O documento está pronto.
• A caixa está pronta.
• Os documentos estão prontos.
• As caixas estão prontas.

It can mean both, depending on context.

In this sentence, pronta most naturally means ready or prepared. But in many situations, Portuguese uses pronto/pronta where English might say done, finished, or ready:

• O jantar está pronto. = Dinner is ready.
• O relatório está pronto. = The report is finished / ready.
• A mala está pronta. = The suitcase is packed / ready.

So the exact English choice depends on what is being talked about.

Yes, A caixa de transporte está preparada is possible, but pronta is usually more natural and more common here.

The difference is roughly:

• pronta = ready
• preparada = prepared

Preparada can sound a little more deliberate or formal, as if someone has actively prepared it. Pronta is the everyday choice for saying something is ready to use.

Usually yes, in normal spoken and written Portuguese, if you mean the specific transport box being discussed.

So A caixa de transporte está pronta sounds natural and complete.

If you remove the article and say Caixa de transporte está pronta, it sounds more like a headline, label, note, or very telegraphic style rather than ordinary conversation.

In Brazilian Portuguese, caixa is usually pronounced roughly KAI-sha.

A simple pronunciation guide for the whole sentence is:

a KAI-sha dji trans-POR-ti es-TA PRON-ta

A few useful notes:

• x in caixa sounds like sh
• está is stressed on the last syllable: es-TA
• pronta has a nasal sound in the first syllable
• de may sound like dji in many Brazilian accents, though pronunciation varies by region

Yes, absolutely. In Brazilian Portuguese, caixa de transporte is very commonly used for a pet carrier, especially for cats and dogs.

But it can also refer more generally to a transport crate, shipping box, or other container used for moving something safely. Context tells you which meaning is intended.

You would say:

As caixas de transporte estão prontas.

Changes:

• a → as
• caixa → caixas
• está → estão
• pronta → prontas

Notice that de transporte stays the same here, because it is functioning as a modifier that names the type/purpose of the boxes.