Learn Hardcore Portuguese (Portugal): A lição de hoje é difícil.

A lição de hoje é difícil. → The lesson today is difficult.
Without the article (Lição de hoje é difícil), the sentence sounds incorrect or very odd in standard Portuguese.

Portuguese normally requires a definite article before singular countable nouns when you are talking about a specific thing that both speaker and listener can identify (here, the lesson scheduled for today).

You could also say:

Esta lição de hoje é difícil. → This lesson today is difficult.

But you still need some kind of determiner (a, esta, essa, etc.). Leaving everything out is not natural.

What gender is lição, and how can I tell it is feminine?

Lição is a feminine noun.

You can tell it is feminine from:

The article: a lição (not o lição).
Its ending: nouns ending in -ção are almost always feminine in Portuguese:
- a nação (the nation)
- a informação (the information)
- a explicação (the explanation)

So you say:

a lição difícil (the difficult lesson)
uma lição difícil (a difficult lesson)

What is the difference between lição and aula?

Both can be translated as lesson, but they are used differently:

a aula = the class session / the class you attend
- Focus on the event in time (time slot with a teacher).
- A aula de hoje é às 10h. → Today’s class is at 10.
a lição = the lesson as content (chapter, exercise set, homework, moral, etc.)
- Focus on what is being taught or a specific exercise/part.
- A lição de hoje é o capítulo 3. → Today’s lesson is chapter 3.

In Portugal:

aula is very common for the class you go to.
lição is common for a specific part of the book, a set of exercises, or a “lesson” in a course.

In Brazil:

People also say a aula de hoje é difícil, especially meaning “today’s class is tough.”
lição is very common in lição de casa (homework), though in Portugal you’ll hear TPC or trabalhos de casa more often.

Why is it de hoje and not do hoje or da hoje?

Hoje is an adverb meaning today, not a noun. Adverbs do not take articles.

de
- noun with article:
  - de
    - o → do (e.g. do livro = of the book)
  - de
    - a → da (e.g. da casa = of the house)

But hoje has no article, so:

de hoje (of today / today’s)
✗ do hoje and ✗ da hoje are incorrect.

You will see the same pattern with other time words used as adverbs:

de ontem (of yesterday)
de amanhã (of tomorrow)

Could I say Hoje a lição é difícil or A lição é difícil hoje instead? Is the word order flexible?

Yes, the word order is somewhat flexible, with small differences in emphasis:

A lição de hoje é difícil.
- Neutral, very natural.
- Slight focus on today’s lesson as a unit: “The lesson for today is difficult.”
Hoje a lição é difícil.
- Emphasis on today: “Today, the lesson is difficult (maybe unlike other days).”
A lição é difícil hoje.
- Grammatically correct, but more marked in everyday speech.
- Can sound like: “The lesson is difficult today (maybe not usually).”

All three are understandable. For a beginner, A lição de hoje é difícil and Hoje a lição é difícil are the most natural choices.

Why is it é difícil and not está difícil? What’s the difference?

É (from ser) and está (from estar) both translate as is, but they express different ideas:

é difícil (ser) → more like a general or inherent characteristic.
- “Today’s lesson is (by nature) difficult / is a difficult one.”
está difícil (estar) → more temporary, situational, or based on current circumstances.
- “Today’s lesson is being difficult (for us right now).”

In practice:

A lição de hoje é difícil.
The content of today’s lesson is objectively hard (e.g., a tough grammar topic).
A lição de hoje está difícil.
Today, we are finding the lesson hard (maybe because people are tired, unprepared, etc.).

Both are possible; the original sentence with é sounds like an objective statement about the lesson itself.

Why doesn’t difícil change to agree with the feminine noun lição? Shouldn’t it be something like difícila?

Adjectives in Portuguese don’t all change in the same way.

Difícil is an adjective that:

Is the same for masculine and feminine:
- uma lição difícil (feminine)
- um teste difícil (masculine)
Changes only for number:
- Singular: difícil
- Plural: difíceis
  - lições difíceis (difficult lessons)
  - testes difíceis (difficult tests)

So there is never a form difícila.

What is the plural of this sentence?

To make the sentence plural, you need to change:

The article: A → As
The noun: lição → lições
The verb: é → são
The adjective: difícil → difíceis

Result:

As lições de hoje são difíceis.
→ Today’s lessons are difficult.

Why is there no word for it in the sentence? In English we say “It is difficult.”

In Portuguese, you normally do not use a separate pronoun like it when the subject is a specific noun that’s already in the sentence.

A lição de hoje é difícil.
Literally: “The lesson of today is difficult.”

Here, a lição de hoje is the subject. There is no need to add ela:

✗ A lição de hoje ela é difícil. (incorrect / very unnatural)

Portuguese is a “null subject” language: the subject pronoun can be omitted when the subject is clear from context, the verb ending, or a full noun phrase.

What do the accent marks in lição and é tell me about pronunciation?

They give you both stress and sound information.

lição
- ç (c-cedilla) is always pronounced like /s/:
  - lição → “li-são” (not “li-Kão”)
- ão is a nasal sound; in European Portuguese it is a nasal diphthong (roughly like “-own” said through the nose).
- The stressed syllable is -ção (the last one).
é
- The accent ´ shows the vowel is stressed and open: /ɛ/, similar to the e in English bed.
- It also distinguishes it from e (without accent):
  - é = is (from ser)
  - e (without accent) = and

So A lição de hoje é difícil roughly (very roughly) sounds like:

[a liˈsɐ̃w dɨ ˈoʒɨ ɛ dɨˈfisil] in European Portuguese.

Is this sentence specifically European Portuguese, or could it also be Brazilian Portuguese?

The sentence A lição de hoje é difícil. is grammatically correct and understandable in both varieties.

Usage differences:

In Portugal, lição for a lesson/topic from a book or course content is very normal.
In Brazil, speakers might more often say:
- A aula de hoje é difícil.
- A aula de hoje está difícil. especially if they mean “today’s class is hard.”

But a Brazilian will certainly understand A lição de hoje é difícil, especially in a school or textbook context.

Could I just say A lição é difícil to mean “Today’s lesson is difficult”?

You can say A lição é difícil, but the meaning is slightly different:

A lição de hoje é difícil.
→ Clearly today’s lesson, specific in time.
A lição é difícil.
→ “The lesson is difficult” in a more general or less specific way.
Context might make it clear that you mean “today’s lesson,” but the sentence by itself doesn’t say today.

If you want to be clear and explicit about today, keep de hoje or another time expression like de hoje de manhã, de hoje à tarde, etc.