Learn Hardcore Portuguese (Portugal): Neste curso, o nível mais difícil é também o mais interessante para mim. - In this course, the most difficult level is also the most interesting for me.

em + este = neste → neste curso = “in this course”
Saying em este curso is grammatically possible but sounds very unnatural and overly formal/old-fashioned in modern European Portuguese.
neste is masculine singular, matching curso, which is also masculine singular.

So Neste curso is simply the normal contracted form of em este curso.

Could I say No curso instead of Neste curso? What’s the difference?

Yes, but the meaning changes slightly.

No curso = em + o curso → “in the course”
- Refers to some course already known from context.
Neste curso = em + este curso → “in this course”
- Points specifically to this course (the one I’m talking about right now, the one I’m on, etc.).

In English you sometimes don’t hear the difference clearly (“in the course” vs “in this course”), but in Portuguese the distinction is explicit and quite important.

Why do we need the article in o nível? Why not just nível mais difícil?

In Portuguese, you normally use the definite article with nouns when you are talking about a specific, identifiable thing.

o nível = “the level”
Without o, nível sounds more like a generic or abstract concept (“level” in general), which is not what you want here.

Also, when you use a relative superlative like o nível mais difícil (“the most difficult level”), the o is part of the standard structure:

o nível mais difícil = “the most difficult level”

Leaving out o (nível mais difícil) sounds incomplete or ungrammatical in this context.

What exactly does mais difícil mean here? Is it “more difficult” or “most difficult”?

mais difícil can mean either “more difficult” (comparative) or “most difficult” (superlative), depending on context and structure.

Here you have:

o nível mais difícil
Literally: “the level more difficult” → understood as “the most difficult level”.

This is called a relative superlative:

o(a) mais + adjective = “the most + adjective”

If you wanted a purely comparative sense (“more difficult than something else”), you’d usually mention what you’re comparing with:

Este nível é mais difícil do que o anterior.
“This level is more difficult than the previous one.”

Why doesn’t difícil change form for masculine/feminine in o nível mais difícil?

The adjective difícil is one of those that have the same form for masculine and feminine in the singular:

o nível difícil (masculine)
a tarefa difícil (feminine)

It only changes in the plural:

os níveis difíceis
as tarefas difíceis

The same happens with many adjectives ending in -e or certain consonants:

interessante → interessante / interessantes
fácil → fácil / fáceis

So in o nível mais difícil, nothing changes for gender; it would only change for number: os níveis mais difíceis.

Why is it é and not está in o nível mais difícil é também o mais interessante?

Portuguese distinguishes between ser and estar:

ser is used for inherent, defining, or stable characteristics.
estar is used for temporary states or conditions.

Here, difícil and interessante are seen as defining qualities of that level within the course. You’re describing what that level is, not just how it happens to be at the moment.

So:

O nível é difícil e interessante. ✔ (defining qualities)
O nível está difícil. → would suggest something like “the level is (unusually) difficult right now”, e.g. because of a specific test or change—more temporary.

Why is também placed after é? Can I move it?

In é também o mais interessante, também means “also / too”. Its position is fairly flexible, and word order changes the emphasis, not the basic meaning:

O nível mais difícil é também o mais interessante.
Neutral, standard; também comes right after the verb.
O nível mais difícil também é o mais interessante.
Very common; slightly more emphasis on “also”.
O nível mais difícil é o mais interessante também.
Puts também at the end, like English “is the most interesting as well”; often a bit more informal.

All three are acceptable in European Portuguese. The version in your sentence is just one of the neutral, standard options.

Why is there another o in o mais interessante? Could I say just é também mais interessante?

The o in o mais interessante is important:

o mais interessante = “the most interesting (one)”
mais interessante (without o) = “more interesting” (comparative, not specifying “the most one”).

In the sentence:

o nível mais difícil é também o mais interessante
literally: “the most difficult level is also the most interesting (level)”.

The second o works like a pronoun, replacing o nível:

o mais interessante = “the most interesting level”.

If you say:

o nível mais difícil é também mais interessante

then you’re saying “the most difficult level is also more interesting”, but more interesting than what isn’t clear; it sounds incomplete without a comparison (e.g. do que os outros).

Why is it para mim and not para eu?

In Portuguese, after prepositions (like para, de, com, por, sem, em), you normally use the oblique (stressed object) pronouns:

eu → mim after prepositions
- para mim = “for me”
- de mim = “of/from me”
- sem mim = “without me”

para eu is used only in very specific grammatical situations, when eu is the subject of a following verb:

Para eu entender, tens de falar mais devagar.
“For me to understand, you have to speak more slowly.”

In your sentence, para mim is not followed by a verb; it just expresses whose opinion it is. So para mim is the correct and natural form.

Could I say a mim instead of para mim?

You can say a mim, but it would change both the preposition and the nuance:

para mim here means “for me / in my opinion”.
a mim usually appears in structures like:
- A mim, ninguém me disse nada.
  “(As for) me, nobody told me anything.”
  It’s more about emphasizing the person who is affected.

In your sentence, the natural preposition for “for me / to me (personally)” is para:

…é também o mais interessante para mim. ✔

Using a mim in this exact sentence would sound odd or overly emphatic in European Portuguese.

Is para mim necessary? What changes if I leave it out?

If you remove para mim, the sentence becomes:

Neste curso, o nível mais difícil é também o mais interessante.

This now sounds like a general statement, as if it were an objective fact about the course.

With para mim, you make it clear that:

This is your personal opinion or experience:
“for me / in my view / as far as I’m concerned”.

So:

with para mim → subjective, personal.
without para mim → sounds more objective or universally true.

Is this sentence specific to European Portuguese, or would Brazilians say it the same way?

This exact sentence is perfectly natural in both European and Brazilian Portuguese:

Neste curso, o nível mais difícil é também o mais interessante para mim.

There is no word here that is region-specific, and the grammar is standard.

Small differences you might hear in Brazil are more about intonation or slightly more frequent placement like:

Neste curso, o nível mais difícil também é o mais interessante pra mim.
(with também é order, and spoken pra instead of para)

But structurally and in writing, it works for both varieties.