Temos que lavar a pia porque o arroz caiu no chão.

Temos que means we have to.

• temos = we have
• que = to in this kind of obligation structure

So:

• Temos que lavar a pia = We have to wash/clean the sink

A very common alternative is:

• Precisamos lavar a pia = We need to wash/clean the sink
• Precisamos de lavar a pia is also possible in some varieties of Portuguese, but in Brazil precisamos lavar is much more natural.

In Brazilian Portuguese, subject pronouns are often omitted when the verb already makes the subject clear.

So both of these are correct:

• Temos que lavar a pia
• Nós temos que lavar a pia

Because temos clearly shows we, the pronoun nós is not necessary.

Including nós can add emphasis, contrast, or clarity, but in normal speech it is often left out.

After ter que + verb, the second verb stays in the infinitive.

Pattern:

• ter que + infinitive

Examples:

• Tenho que estudar = I have to study
• Temos que sair = We have to leave
• Ela tem que trabalhar = She has to work

So:

• Temos que lavar a pia
  not
• Temos que lavamos a pia

The infinitive is the normal form after this structure.

Portuguese uses definite articles much more often than English does.

So where English might say:

• wash the sink
• rice fell on the floor

Portuguese naturally says:

• lavar a pia
• o arroz caiu no chão

Here:

• a = the for a feminine singular noun
• o = the for a masculine singular noun

This is very normal in Portuguese, even in places where English might omit the.

In Brazilian Portuguese, pia usually means sink, especially in everyday language.

Most often:

• pia da cozinha = kitchen sink
• pia do banheiro = bathroom sink

In this sentence, lavar a pia most naturally means to wash/clean the sink.

Depending on region and context, people may also use other words, but pia is very common and natural in Brazil.

Both can make sense, but they are slightly different.

• lavar = to wash
• limpar = to clean

In real life, Portuguese often uses lavar in situations where English might more naturally say clean, especially if water is involved.

So:

• lavar a pia = literally wash the sink
• limpar a pia = clean the sink

In this sentence, lavar a pia sounds natural because the rice fell and the sink needs to be washed.

Here, porque means because, so it is written as one word.

• porque = because

Examples:

• Fiquei em casa porque choveu.
• Temos que lavar a pia porque o arroz caiu no chão.

By contrast:

• por que is usually used in questions, like why?
  • Por que você saiu? = Why did you leave?

So in your sentence, porque is correct because it introduces the reason.

Caiu is the preterite form of the verb cair (to fall).

• cair = infinitive
• caiu = fell

This is the 3rd person singular preterite, which is used for a completed action in the past.

So:

• o arroz caiu no chão = the rice fell onto/on the floor

Other forms of cair in the preterite:

• eu caí = I fell
• você/ele/ela caiu = you/he/she fell
• nós caímos = we fell

A detail learners often notice: eu caí has an accent, but ele caiu does not.

Because Portuguese contracts em + o into no.

• em = in / on / at
• o = the
• em + o = no

So:

• no chão = on the floor / onto the floor

Other common contractions:

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

Examples:

• na mesa = on the table
• no carro = in the car
• nas mãos = in the hands

These contractions are standard and required in normal Portuguese.

In this sentence, it can correspond to English on the floor, but the full idea is really that the rice fell to the floor.

Portuguese often uses cair no chão where English might say:

• fall on the floor
• fall onto the floor
• fall to the floor

So no chão is the normal Portuguese expression here, even if the best English translation changes a little depending on context.

Because chão is a masculine noun in Portuguese, so it takes o:

• o chão = the floor / the ground

There is not always a logical reason a noun is masculine or feminine; this is something you usually learn together with the word.

So it is best to memorize nouns with their article:

• a pia
• o arroz
• o chão

That makes it much easier to build correct sentences later.

The sentence follows a very normal Portuguese word order:

• Temos que lavar a pia = main idea
• porque o arroz caiu no chão = reason

So the structure is:

• [We have to wash the sink] because [the rice fell on the floor].

This is very similar to English word order, which makes the sentence fairly straightforward for English speakers.

You could also move parts around for emphasis in some contexts, but this version is the most neutral and natural.

Yes. A few natural alternatives are:

• Temos que limpar a pia porque o arroz caiu no chão.
• Precisamos lavar a pia porque o arroz caiu no chão.
• Precisamos limpar a pia porque o arroz caiu no chão.

The differences are small:

• temos que = have to
• precisamos = need to
• lavar = wash
• limpar = clean

All of these are natural, but your original sentence is completely correct and idiomatic.