O vencimento do boleto é amanhã, mas eu ainda posso pagar a primeira parcela hoje.

Because Portuguese usually contracts de + o into do.

So:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

That means o vencimento do boleto literally comes from o vencimento de o boleto, but the contracted form do is the correct natural form.

Portuguese often uses the present tense to talk about a near future event, especially when it is fixed or scheduled. So é amanhã is very natural in everyday Brazilian Portuguese.

Both are possible:

• O vencimento do boleto é amanhã. = very natural, conversational
• O vencimento do boleto será amanhã. = also correct, a bit more formal or explicit

Here ainda means still. It shows that there is still time or possibility left: I can still pay... today.

Ainda can have a few meanings depending on context, such as:

• still
• yet
• even

But in eu ainda posso pagar, the best meaning is still.

Because poder is followed directly by an infinitive verb in Portuguese, just like can + verb in English.

So:

• posso pagar = I can pay
• posso fazer = I can do
• posso ir = I can go

Also, pagar does not need a preposition before its direct object here, so pagar a primeira parcela is the normal structure.

Parcela means an installment: one part of a total amount that is paid over time. So a primeira parcela means the first installment.

This is very common in Brazilian Portuguese when talking about paying something in parts:

• pagar em parcelas = pay in installments
• primeira parcela = first installment
• última parcela = last installment

Portuguese uses definite articles much more often than English. In this sentence, a primeira parcela refers to a specific, known installment, so the article a sounds natural.

In normal speech, leaving it out would usually sound incomplete or unnatural here. English often says the first installment, and Portuguese does the same: a primeira parcela.

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

So both are possible:

• mas eu ainda posso pagar...
• mas ainda posso pagar...

The version with eu gives a little more emphasis or contrast, which fits well after mas.

Yes. A very common shorter version is:

• O boleto vence amanhã.

This literally means The boleto is due tomorrow. It is often more natural in everyday speech. The original sentence is also correct, but it sounds a bit more explicit or formal because it focuses on the due date itself.

Yes, Portuguese word order is somewhat flexible, but some positions sound more natural than others.

The original version is very natural:

• mas eu ainda posso pagar a primeira parcela hoje

You could also say:

• mas hoje eu ainda posso pagar a primeira parcela

That puts more emphasis on today.
But ainda usually stays close to the verb phrase, so eu ainda posso pagar sounds better than less natural alternatives like eu posso ainda pagar.

The tricky part is that amanhã has both a nasal vowel and nh.

A rough English-friendly approximation is:

• ah-ma-NYÃ

A few details:

• nh sounds like the ny in canyon
• ã is nasal, so the sound comes partly through the nose
• the stress falls on the last syllable

So the end is not a plain -na sound; it is a nasal -nhã.