Na quarta-feira, eu fico em casa e organizo o estojo antes da aula.

Na is a contraction of em + a.

• em = in / on / at
• a = the

So na quarta-feira literally contains in/on the Wednesday, and in natural English we translate that as on Wednesday.

A useful European Portuguese note:

• na quarta-feira often sounds like on Wednesday in the sense of a particular Wednesday.
• For a repeated habit like on Wednesdays, European Portuguese often prefers à quarta-feira or às quartas-feiras.

So the form in your sentence is perfectly normal, but the exact nuance depends on context.

Because in Portuguese, the days of the week are normally written in lowercase:

• segunda-feira
• terça-feira
• quarta-feira
• quinta-feira
• sexta-feira

This is different from English, where Wednesday is capitalized.

The phrase Na quarta-feira is an introductory time expression, and Portuguese often separates that kind of phrase with a comma.

So the comma helps mark:

• When? → Na quarta-feira
• What happens? → eu fico em casa e organizo o estojo antes da aula

In a short sentence, some writers might omit the comma, but with it, the sentence feels clear and natural.

You could leave it out.

Portuguese often drops subject pronouns because the verb ending already shows who the subject is:

• fico = I stay / I am staying
• organizo = I organize / I am organizing

So this is also natural:

• Na quarta-feira, fico em casa e organizo o estojo antes da aula.

Including eu can add:

• emphasis
• contrast
• clarity

For example, if you want to stress I rather than someone else, eu is useful.

Because ficar and estar are not the same.

• estar em casa = to be at home
• ficar em casa = to stay at home / remain at home

In your sentence, the idea is not just location, but the action or choice of staying home. That is why fico em casa is natural here.

So:

• Estou em casa. = I am at home.
• Fico em casa. = I stay home / I remain at home.

Em casa is the normal fixed expression for at home.

So:

• Estou em casa. = I am at home.
• Fico em casa. = I stay at home.

By contrast, na casa usually means in the house or at the house, often referring to a specific house:

• Estou na casa da minha avó. = I am at my grandmother’s house.

So in your sentence, em casa is the correct idiomatic choice.

Fico is the 1st person singular present indicative of ficar.

That means:

• eu fico = I stay / I remain / I am staying

A few forms of ficar in the present:

• eu fico
• tu ficas
• ele/ela fica
• nós ficamos
• vocês/eles/elas ficam

In European Portuguese, tu is very common in informal speech, so it is useful to learn that pattern too.

Organizo is the 1st person singular present indicative of organizar.

So:

• eu organizo = I organize / I am organizing

It matches the same subject as fico, so the sentence has two coordinated verbs with the same subject:

• eu fico
• (eu) organizo

That is why Portuguese does not need to repeat eu before the second verb.

In this school-related sentence, estojo most likely means pencil case.

More generally, estojo can mean a case or container for small items, depending on context, such as:

• a pencil case
• a makeup case
• a small storage case

But with antes da aula and the classroom context, pencil case is the most natural interpretation.

Because da is a contraction of de + a.

The expression is antes de = before.

When de comes before the feminine singular article a, they contract:

• de + a = da

So:

• antes da aula = before the class / before class

This contraction is normal and expected.

Compare:

• antes da aula = before class
• antes de sair = before leaving

With a verb, you keep de:

• antes de estudar
• antes de entrar

With a noun that has the article a, you get da:

• antes da aula
• antes da reunião

Aula usually means class, lesson, or teaching session.

It does not mean the school building or classroom in general.

So:

• antes da aula = before class
• durante a aula = during class
• depois da aula = after class

In this sentence, it refers to the lesson or class period that comes later.

It can do either, depending on context, but in this sentence it most naturally sounds like a routine or regular arrangement.

Portuguese present tense often works like English present simple:

• Na quarta-feira, eu fico em casa...
  = On Wednesday, I stay at home...

Because the sentence includes a time expression and describes a sequence of actions, it sounds very much like something the speaker normally does on that day.

Grammatically, it most naturally connects to the nearest verb phrase:

• organizo o estojo antes da aula

So the clearest reading is:

• I organize the pencil case before class.

However, depending on context, a listener may also understand the whole situation as happening in that time frame.

If you want before class to clearly modify everything, you could move it forward:

• Antes da aula, eu fico em casa e organizo o estojo.

That placement makes the time reference feel broader.