Se eu pagar a mensalidade hoje, posso começar o curso amanhã.

After se when Portuguese talks about a real future possibility, it often uses the future subjunctive.

So:

• Se eu pagar... = If I pay...
• Quando eu pagar... = When I pay...

For regular -ar verbs like pagar, the future subjunctive form for eu looks the same as the infinitive:

• pagar

That is why Se eu pagar is correct here.

Compare:

• Se eu pagar a mensalidade hoje, posso começar amanhã.
• Not: Se eu pagarei...

Portuguese does not normally use the future tense right after se in this kind of conditional sentence.

Se means if.

It introduces the condition:

• Se eu pagar a mensalidade hoje = If I pay the monthly fee today

This sets up the condition for the second part:

• posso começar o curso amanhã = I can start the course tomorrow

So the full structure is:

• If X, then Y

It is not always strictly necessary, but here it is very natural and helpful.

Portuguese often drops subject pronouns because the verb ending already shows the person. But with the future subjunctive, some forms can look identical across persons, so speakers often include the pronoun for clarity.

So both are possible:

• Se eu pagar a mensalidade hoje...
• Se pagar a mensalidade hoje...

But the version with eu is clearer and more neutral, especially for learners.

Mensalidade is a monthly fee, monthly payment, or sometimes monthly tuition installment, depending on context.

In this sentence, it likely refers to the payment required for the course.

Related words:

• mensalidade = monthly fee/payment
• taxa = fee
• pagamento = payment
• anuidade = annual fee
• matrícula = enrollment/registration fee or enrollment itself, depending on context

So a mensalidade is not just any payment; it specifically suggests a recurring monthly charge.

Portuguese uses definite articles much more often than English.

So where English might say:

• pay tuition
• start the course

Portuguese commonly says:

• pagar a mensalidade
• começar o curso

The articles a and o are normal and expected here.

They agree with the nouns:

• mensalidade is feminine singular → a
• curso is masculine singular → o

Posso começar literally means I can start, but in Portuguese the present tense is often used for a future situation when the time is already clear from context.

Since the sentence already says:

• hoje
• amanhã

the timing is obvious.

So:

• posso começar o curso amanhã = I can start the course tomorrow

is perfectly natural.

You could also say:

• poderei começar o curso amanhã

which sounds a bit more formal or explicit: I will be able to start the course tomorrow.

Both are correct, but posso começar is very natural in everyday speech.

Posso can express more than one idea, and here it most likely means permission or practical possibility.

Depending on context, it could mean:

• Am I allowed to start tomorrow?
• Is it possible for me to start tomorrow?

In many real-life situations, those ideas overlap. If paying today satisfies the requirement, then starting tomorrow becomes possible/allowed.

So posso here is best understood as can / be able to / be allowed to, depending on context.

The comma separates the conditional clause from the main clause:

• Se eu pagar a mensalidade hoje, posso começar o curso amanhã.

This is very standard punctuation in Portuguese, just as in English with a fronted if clause:

• If I pay today, I can start tomorrow.

The comma helps the sentence read more clearly.

Yes, the word order can change, although some versions sound more natural than others.

Possible versions:

• Se eu pagar a mensalidade hoje, posso começar o curso amanhã.
• Se eu pagar hoje a mensalidade, posso começar o curso amanhã.

Both are understandable. The original version sounds more neutral and natural because a mensalidade stays closely connected to pagar.

Portuguese word order is somewhat flexible, but not infinitely so. Learners should usually prefer the more standard order unless there is a reason to emphasize something.

Because começar can be used directly with a direct object.

So:

• começar o curso = start the course

This is the most straightforward way to say it.

You may also hear other structures in different contexts, such as:

• começar no novo emprego = start at the new job
• começar a aula = start the class
• começar no curso can appear in some spoken contexts, but começar o curso is the standard and best choice here.

This is an important distinction.

Se eu pagar..., posso...

• real or likely future possibility
• If I pay..., I can...

Se eu pagasse..., poderia...

• hypothetical, less direct, more uncertain, or contrary-to-fact
• If I paid..., I could...

So:

• Se eu pagar a mensalidade hoje, posso começar o curso amanhã. = This sounds like a real, practical possibility.

• Se eu pagasse a mensalidade hoje, poderia começar o curso amanhã. = This sounds more hypothetical, like discussing a scenario.

The first one is the natural choice when someone is talking about an actual plan.

In Brazilian Portuguese, it is roughly pronounced:

• men-sa-li-DA-de

The stress falls on:

• da

A rough English-style approximation is:

• men-sah-lee-DAH-jee

A few helpful notes:

• men has a nasal quality in Portuguese
• the d in dade often sounds a bit like j in many Brazilian accents before i/e
• the final e is usually pronounced, not silent

So in many Brazilian accents it sounds close to:

• men-sah-lee-DAH-jee

Yes. That is completely correct.

Portuguese often omits subject pronouns, but you can include them for:

• emphasis
• clarity
• rhythm

So both are natural:

• Se eu pagar a mensalidade hoje, posso começar o curso amanhã.
• Se eu pagar a mensalidade hoje, eu posso começar o curso amanhã.

The version without the second eu is slightly more compact and is probably the most neutral.

Yes, to some extent.

These are all possible:

• posso começar o curso amanhã
• amanhã posso começar o curso
• posso amanhã começar o curso (possible, but less natural in many contexts)

The most natural version here is:

• posso começar o curso amanhã

It keeps the sentence clear and follows a very common rhythm in Portuguese.