Learn Hardcore Portuguese (Portugal): Caso eu esteja muito cansado amanhã, vou ficar em casa a estudar português. - If I am very tired tomorrow, I will stay at home studying Portuguese.

caso is a bit more formal/literary and clearly calls for the subjunctive (conjuntivo).
- Caso eu esteja muito cansado amanhã…
se is more neutral and very common in everyday speech; in this type of sentence it also normally uses the subjunctive:
- Se eu estiver muito cansado amanhã…

In European Portuguese:

Both are correct here:
- Caso eu esteja muito cansado amanhã, vou ficar em casa…
- Se eu estiver muito cansado amanhã, vou ficar em casa…
se is more common in informal speech; caso can sound a bit more careful, formal, or “written”.

So the choice of caso here mainly affects tone/register, not the basic meaning.

Why is it esteja and not estou or estarei?

esteja is the present subjunctive (presente do conjuntivo) of estar.

In Portuguese, many if-clauses that talk about a possible future situation use the subjunctive:

Caso eu esteja muito cansado amanhã…
= If I am very tired tomorrow…

Forms:

eu estou – present indicative (a factual statement: I am)
eu estarei – future indicative (I will be)
eu esteja – present subjunctive (used after words like caso, se, quando in uncertain or hypothetical situations)

Patterns to remember:

Caso / Se / Quando + subjunctive to talk about a possible future condition:
- Se eu estiver / Caso eu esteja cansado amanhã, vou ficar em casa.

Using estou here (caso eu estou) would be ungrammatical; the conjunction caso requires the subjunctive in this kind of conditional.

Could I say Se eu estiver muito cansado amanhã instead of Caso eu esteja muito cansado amanhã? Is there a difference?

Yes, you can say both:

Caso eu esteja muito cansado amanhã…
Se eu estiver muito cansado amanhã…

Differences:

Grammar: both take the subjunctive, and both are correct.
Register / style:
- Se eu estiver… is the most common, neutral choice in everyday Portuguese.
- Caso eu esteja… sounds more formal, careful, or written; it can feel slightly more “bookish”.

Meaning-wise, for this sentence they are practically the same.

Can I drop the eu and just say Caso esteja muito cansado amanhã?

Yes, you can:

Caso eu esteja muito cansado amanhã…
Caso esteja muito cansado amanhã…

Portuguese often omits subject pronouns when the verb ending already shows the person:

(Eu) esteja – only 1st person singular in this tense, so it’s clear it means I.

However:

In speech, learners might want to keep eu for clarity and practice.
Native speakers may add eu for emphasis or contrast:
- Caso eu esteja muito cansado, tu vais sozinho.

So omitting eu is grammatically fine here.

Why is there a comma before vou ficar em casa…? Is that obligatory?

Yes, the comma is standard here:

Caso eu esteja muito cansado amanhã, vou ficar em casa a estudar português.

Reason:

Caso eu esteja muito cansado amanhã is a subordinate clause (the condition).
vou ficar em casa a estudar português is the main clause (the result).
When the subordinate clause comes before the main clause, Portuguese normally uses a comma to separate them.

If you reverse the order, you normally don’t use a comma:

Vou ficar em casa a estudar português caso eu esteja muito cansado amanhã. (no comma)

Why vou ficar and not ficarei? Are both correct?

Both are grammatically correct:

Vou ficar em casa a estudar português.
Ficarei em casa a estudar português.

Differences:

vou ficar = ir (present) + infinitive
- Very common in spoken European Portuguese.
- Sounds natural and conversational.
ficarei = simple future tense
- Used more in writing, official texts, or when you want a slightly more formal or emphatic tone.
- In everyday speech, Europeans often avoid it for regular conversation.

So your sentence uses the most natural, spoken-style future.

What exactly does ficar em casa a estudar português mean? Is it “stay” or “become”?

ficar is very flexible. Here it means to stay / remain:

ficar em casa = to stay at home
ficar em casa a estudar português = stay at home (and be) studying Portuguese

Structure:

ficar + a + infinitive
- Focuses on remaining in a place or state doing an activity:
  - Vou ficar em casa a estudar. – I’ll stay at home studying.
  - Fiquei a trabalhar até tarde. – I stayed working until late.

Other common meanings of ficar (for context):

ficar + adjective: become / end up
- Fiquei cansado. – I became tired / I ended up tired.
ficar em + place: be located / end up staying
- O hotel fica perto do centro. – The hotel is (located) near the centre.

In your sentence, it’s clearly “stay (at home)”.

Why is it em casa and not na casa?

em casa is a fixed expression meaning at home:

Vou ficar em casa. – I’m going to stay at home.

You normally do not use an article here (a casa) unless you’re specifying which house:

em casa – at home (general)
na casa do João – at João’s house
na minha casa – at my house

So:

Vou ficar em casa. ✔ (natural)
Vou ficar na casa. ❌ (strange unless previously defined: I’ll stay in the house – which one?)

What does a estudar mean? Why not just estudar or para estudar?

a estudar here is “a + infinitive”, which in European Portuguese often marks an ongoing activity, similar to English “(be) doing”:

ficar a estudar ≈ stay studying
estou a estudar ≈ I am studying

Differences:

ficar em casa a estudar português
- Stresses the ongoing activity of studying while you stay at home.
ficar em casa a estudar is much more natural than ficar em casa para estudar in EP in this context.
- para estudar tends to focus on purpose (in order to study), and with ficar it often sounds less idiomatic.

Compare:

Vou para a biblioteca estudar. – I’m going to the library (in order) to study.
Vou ficar em casa a estudar. – I’ll stay at home (and be) studying.

So ficar … a estudar is the usual EP construction for “stay … studying”.

Why not use the -ndo form, like estudando, as in Brazilian Portuguese?

In European Portuguese, the gerúndio (-ndo forms like estudando) exists but is much less used, especially in the progressive:

EP natural: Vou ficar em casa a estudar português.
BP natural: Vou ficar em casa estudando português.

In EP:

estar a + infinitive is the standard progressive:
- Estou a estudar. – I am studying.
Using estudando here (vou ficar em casa estudando português) would sound very Brazilian or non‑native in European usage, except in some set expressions.

So in Portugal, prefer ficar a estudar, estar a estudar, etc., rather than ficar estudando, estar estudando.

Why is português not capitalized? In English “Portuguese” is.

Capitalization rules differ:

In English, names of languages and nationalities are capitalized:
- Portuguese, English, French…
In Portuguese, language names are not capitalized:
- português, inglês, francês (for the languages)

But nationalities as adjectives or nouns referring to people are capitalized:

um Português – a Portuguese man
uma Portuguesa – a Portuguese woman
um aluno português – a Portuguese student (adjective often lowercase in modern usage, but the noun for the person is capitalized in formal writing)

In a estudar português, it’s the language, so it stays lowercase.

Why muito cansado and not muito cansada? How does agreement work here?

cansado/cansada is an adjective and must agree with the gender and number of the subject:

If eu = a man:
- Caso eu esteja muito cansado amanhã…
If eu = a woman:
- Caso eu esteja muito cansada amanhã…

Plural:

If nós = a group of men or mixed:
- Caso estejamos muito cansados amanhã…
If nós = only women:
- Caso estejamos muito cansadas amanhã…

So in your sentence, muito cansado assumes the speaker is male. For a female speaker, you’d say muito cansada.