A professora de inglês disse que a próxima mensalidade deve ser paga na sexta-feira.

Portuguese often uses the definite article with specific people or roles in a way that English often does not.

So A professora de inglês is the natural way to say the English teacher.

In Portuguese, leaving out the article here would usually sound incomplete or unnatural in a normal sentence. Articles are very common before nouns, including professions and titles when you are talking about a specific person.

In Portuguese, names of languages, nationalities, days of the week, and months are normally not capitalized.

So:

• inglês = English
• brasileiro = Brazilian
• sexta-feira = Friday

This is different from English, where those words are capitalized.

Because these mean different things:

• professora de inglês = a teacher of English / an English teacher
• professora inglesa = an English teacher in the sense that she is from England or is English by nationality

So de inglês tells you what subject she teaches, not where she is from.

Mensalidade usually means a monthly fee, monthly payment, or monthly tuition installment.

It is commonly used for things like:

• school tuition
• course fees
• gym memberships
• condo or association fees

It is more specific than just payment. It refers to something charged every month.

In Portuguese, many adjectives can come either before or after the noun, but some are much more natural in one position than the other.

With time-sequence words like próximo/próxima, it is very common to put them before the noun:

• próxima aula
• próxima semana
• próxima mensalidade

Saying mensalidade próxima would sound unusual here. So a próxima mensalidade is the normal phrasing.

Because this is ordinary reported speech.

Disse que means said that, and what follows is presented as information or a statement, so Portuguese normally uses the indicative, not the subjunctive.

The subjunctive would be more likely after expressions of:

• doubt
• desire
• emotion
• uncertainty
• necessity introduced in certain ways

But disse que simply reports what was said, so deve ser paga is normal.

It is a passive construction:

• deve = must / should / is supposed to
• ser = to be
• paga = paid

So the structure is:

deve + ser + past participle

This is like English must be paid or should be paid.

The sentence focuses on the fee, not on the person who will pay it.

Because in Portuguese, the past participle in a passive construction agrees with the noun it describes.

Here the noun is mensalidade, which is:

• feminine
• singular

So the participle must also be feminine singular:

• mensalidade paga

Compare:

• o boleto deve ser pago = masculine singular
• a mensalidade deve ser paga = feminine singular
• as mensalidades devem ser pagas = feminine plural

It can mean either, depending on context.

Dever can express:

• obligation
• expectation
• recommendation

In a sentence about a payment deadline, deve ser paga often feels stronger than a mild English should. Depending on context, it may be understood more like:

• must be paid
• is supposed to be paid
• is due

So deve is somewhat flexible, and the exact force comes from the situation.

Na is the contraction of:

• em
  • a = na

With days of the week, Portuguese often uses em plus the article:

• na segunda-feira
• na terça-feira
• na sexta-feira

So na sexta-feira literally comes from em a sexta-feira, but that form contracts to na sexta-feira.

Here it normally means one specific Friday, because the sentence is talking about a próxima mensalidade, a specific upcoming payment.

If you wanted to say every Friday or on Fridays, Portuguese would usually say:

• às sextas-feiras

So the contrast is:

• na sexta-feira = on Friday / on a specific Friday
• às sextas-feiras = on Fridays / every Friday

Also, sexta-feira has a hyphen because Portuguese writes weekday names like this:

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