Eu comprei outra régua porque a antiga ficou no estojo da minha sobrinha.

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

So both of these are natural:

• Eu comprei outra régua...
• Comprei outra régua...

Including Eu can add a little emphasis, contrast, or clarity. In this sentence, Eu is not required, but it is perfectly normal.

Comprei is the pretérito perfeito of comprar.

It refers to a completed action in the past: I bought.

• compro = I buy / I am buying
• comprava = I used to buy / I was buying
• comprei = I bought

Here it shows a single finished action.

Outra means another or a different one.
Nova means new.

So:

• outra régua = another ruler
• uma régua nova = a new ruler

In this sentence, the speaker bought one more ruler because the previous one was elsewhere. The important idea is replacement, not necessarily that it is brand-new.

Because régua is a feminine singular noun, and adjectives/determiners that go with it must agree.

• outro livro = another book
• outra régua = another ruler

The same thing happens later with a antiga: it is feminine singular too, because it refers back to régua.

Portuguese often leaves out a noun when it is already understood.

So a antiga means:

• the old one
• the previous one

The omitted noun is régua.

This is very common:

• Prefiro a azul. = I prefer the blue one.
• A minha é maior. = Mine is bigger.
• Comprei outra, porque a antiga... = I bought another one because the old one...

Here it most naturally means previous or the one I had before.

In many contexts, antigo/antiga can mean:

• old
• former
• previous

In this sentence, a antiga is best understood as the previous ruler.

A learner might compare it with velha:

• a régua velha = the old/worn-out ruler
• a régua antiga = the old/former/previous ruler

In real usage, antiga often sounds better when contrasting an old item with a replacement.

Because here it means because, introducing the reason.

• porque = because

This is different from:

• por que = why / for which reason
• por quê = why? at the end of a sentence
• porquê = the reason

So in this sentence, porque is correct because it explains why the speaker bought another ruler.

Not here.

Ficar can mean several things, including:

• to stay
• to remain
• to be left
• to end up
• to become

In this sentence, ficou no estojo da minha sobrinha means something like:

• was left in my niece’s pencil case
• ended up in my niece’s pencil case
• stayed in my niece’s pencil case

So it does not mean became here.

Because the meanings are slightly different.

• ficou suggests the ruler ended up there / was left there
• estava would simply describe where it was

Compare:

• A antiga ficou no estojo da minha sobrinha.
  = The old one got left in my niece’s pencil case / ended up there.

• A antiga estava no estojo da minha sobrinha.
  = The old one was in my niece’s pencil case.

The original sentence gives more of a reason for buying another one: it was left there, so the speaker needed a replacement.

Because Portuguese normally contracts em + o into no.

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

So:

• no estojo = in the pencil case

This kind of contraction is extremely common in Portuguese.

In this context, estojo means a pencil case or school supply case.

It is the kind of case where someone might keep:

• pencils
• pens
• erasers
• a ruler

So the sentence strongly suggests a school-related or child-related context.

Because da is the contraction of de + a.

• de + a = da

So:

• da minha sobrinha = of my niece / my niece’s

This is how Portuguese often expresses possession.

Also, in Brazilian Portuguese, it is very common to use the definite article before a possessive:

• a minha sobrinha
• o meu irmão
• a minha mãe

So da minha sobrinha is completely natural.

Yes, grammatically it could. But in Brazilian Portuguese, da minha sobrinha is usually more natural in everyday speech.

Compare:

• o estojo da minha sobrinha = very common
• o estojo de minha sobrinha = possible, but often sounds more formal or literary

So the version in the sentence is the one most learners should expect to hear most often.

In Brazilian Portuguese, régua is pronounced roughly like REH-gwa, with the stress on the first syllable.

A rough guide:

• ré = like reh
• gua = like gwa

So: REH-gwa

The accent mark in régua shows the stressed syllable is ré.

Because the main noun being discussed is régua, and régua is feminine.

That affects the words connected to it:

• outra régua
• a antiga

And sobrinha is also feminine because it means a female niece.

So the sentence contains feminine forms for two different reasons:

1. régua is a feminine noun
2. sobrinha refers to a female person

Normally, sobrinha means niece.

• sobrinho = nephew
• sobrinha = niece

So in this sentence, it refers specifically to a female niece.