A caixa de areia está limpa.

A is the feminine singular definite article, meaning the.

It is used because caixa is a feminine singular noun, so:

• a caixa = the box
• uma caixa = a box

In this sentence, a suggests we are talking about a specific box that both speaker and listener can identify.

Yes. Caixa is a feminine noun, so it takes feminine forms around it:

• a caixa
• a caixa limpa

That is why both the article a and the adjective limpa are feminine singular.

Literally, it means box of sand or sand box.

In real usage, caixa de areia can refer to different things depending on context, such as:

• a sandbox
• a litter box

Portuguese often uses the structure noun + de + noun where English might use a compound noun.

Because de areia expresses the type or purpose of the box, not possession of some specific sand.

• caixa de areia = a sand box / litter box / box for sand
• caixa da areia = the box of the sand / the sand’s box, referring to some specific sand

So de areia is the normal form for naming this kind of object.

Portuguese usually uses estar for a temporary condition or current state, and ser for a more permanent characteristic.

Here, clean is understood as the box’s condition right now, so está is the natural choice:

• está limpa = it is clean right now / it is currently clean

If you said é limpa, it would sound more like a general characteristic, such as it is a clean kind of box or it is generally clean, which is less natural in this context.

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

Since caixa is feminine singular, the adjective must also be feminine singular:

• masculine singular: limpo
• feminine singular: limpa
• masculine plural: limpos
• feminine plural: limpas

So:

• o quarto está limpo
• a caixa está limpa

The accent in está shows the stressed syllable and helps distinguish it from esta.

• está = is
• esta = this (feminine)

So the accent is important both for pronunciation and for meaning.

A simple approximate pronunciation for Brazilian Portuguese is:

a KAI-sha dji a-REI-a es-TA LEEM-pa

A few useful notes:

• caixa sounds roughly like KAI-sha
• areia sounds roughly like a-REI-a
• está is stressed on the last syllable: es-TA
• limpa sounds roughly like LEEM-pa

Depending on the Brazilian accent, de may sound more like dji or dee in fast speech.

Yes, if the context already makes it clear what you are talking about.

Portuguese often omits the subject when it is understood. So if everyone already knows you mean the litter box or sandbox, Está limpa is perfectly natural and means It’s clean.

But if you are introducing the topic or need to be clear, A caixa de areia está limpa is better.

Normally, no. In standard Portuguese, you would usually keep the article:

• A caixa de areia está limpa

Leaving it out can sound unnatural in ordinary speech, though you might see article omission in:

• headlines
• labels
• notes
• very telegraphic style

So for normal conversation, keep a.

The plural is:

As caixas de areia estão limpas.

Here is what changes:

• a → as
• caixa → caixas
• está → estão
• limpa → limpas

Everything agrees with the plural feminine noun.