No curso de inglês, a mensalidade é paga todo mês.

No is the contraction of em + o.

• em = in / on / at
• o curso = the course

So:

• em o curso → no curso

This is mandatory in normal Portuguese. You would not usually say em o curso.

So No curso de inglês means In the English course or For the English course, depending on context.

In Portuguese, this kind of relationship is usually expressed with de.

• curso de inglês = English course
• literally: course of English

This is very common:

• aula de português = Portuguese class
• livro de história = history book
• professor de matemática = math teacher

Using de is the normal way to link the main noun to the subject area.

In Portuguese, names of languages are normally written with a lowercase letter, not a capital letter.

So:

• inglês = English
• português = Portuguese
• espanhol = Spanish

This is different from English, where language names are capitalized.

Mensalidade means a monthly fee or monthly payment.

It is often used for things like:

• courses
• schools
• gyms
• clubs
• subscription-like services

So in this sentence, a mensalidade is the amount you have to pay each month for the English course.

It is not exactly the same as tuition in every context, although sometimes that may be the best translation.

Portuguese uses definite articles more often than English.

Here, a mensalidade refers to the specific monthly fee for that course. So the article sounds natural and expected.

Compare:

• A mensalidade é paga todo mês. = The monthly fee is paid every month.
• Mensalidade é paga todo mês. = possible only in very specific styles, but it sounds less natural here.

In normal Portuguese, a mensalidade is the natural choice.

This is a passive structure.

• é = is
• paga = paid

So a mensalidade é paga means the monthly fee is paid.

Portuguese often uses this kind of passive, just like English:

• O livro é lido. = The book is read.
• A conta é paga. = The bill is paid.

In your sentence, the focus is on the fee itself, not on who pays it.

Because paga agrees with mensalidade, which is a feminine singular noun.

• o pagamento é feito
• a mensalidade é paga

Agreement matters in this kind of passive construction:

• masculine singular: pago
• feminine singular: paga
• masculine plural: pagos
• feminine plural: pagas

Since mensalidade is feminine, the correct form is paga.

Todo mês means every month.

This is a very important pattern:

• todo + singular noun = every
  • todo dia = every day
  • toda semana = every week
  • todo mês = every month

So here, todo mês tells you how often the fee is paid.

They mean different things.

• todo mês = every month
• todo o mês = the whole month / all month long

This is a very common learner issue.

Examples:

• Eu pago todo mês. = I pay every month.
• Choveu todo o mês. = It rained the whole month.

So in your sentence, todo mês is correct because the meaning is repeated frequency, not duration.

Yes, but todo mês is usually more natural in everyday Brazilian Portuguese.

• todo mês = very common, natural
• cada mês = possible, but often a bit more emphatic or context-dependent

You might also hear:

• mensalmente = monthly

So these are all possible, but they do not sound equally natural in all situations:

• A mensalidade é paga todo mês. = very natural
• A mensalidade é paga a cada mês. = understandable, a bit less natural here
• A mensalidade é paga mensalmente. = correct, a bit more formal

The comma separates the introductory phrase from the rest of the sentence.

• No curso de inglês, a mensalidade é paga todo mês.

This is similar to English when you begin with a phrase like In this course, ...

The comma is helpful and natural here, especially because No curso de inglês sets the context first.

Without the comma, the sentence is still understandable:

• No curso de inglês a mensalidade é paga todo mês.

But the version with the comma is clearer and more polished.

Yes, it can feel a bit repetitive, especially to an English speaker.

That is because:

• mensalidade already suggests something monthly
• todo mês also means every month

Even so, Portuguese can still use both together for clarity or emphasis. It is not wrong.

Depending on context, more compact alternatives could be:

• No curso de inglês, a mensalidade é paga mensalmente.
• No curso de inglês, paga-se mensalidade.
• No curso de inglês, o pagamento é mensal.

But the original sentence is perfectly understandable and natural enough.

Curso usually means a course, program, or series of classes, not just one individual lesson.

So:

• curso de inglês = English course/program
• aula de inglês = English class/lesson

If you mean one class meeting, aula is the better word. If you mean the overall thing someone enrolls in, curso is the better word.