Eu não entendi esta parte da lição.

Yes, you can often leave eu out in Portuguese because the verb form entendi already shows that the subject is I.

So both of these are natural:

• Eu não entendi esta parte da lição.
• Não entendi esta parte da lição.

Including eu can add clarity, emphasis, or contrast, such as I didn’t understand it, but maybe someone else did.

In Portuguese, não normally goes directly before the verb it negates.

• Eu entendi = I understood
• Eu não entendi = I did not understand

This is the standard word order in simple sentences.

Entendi is the preterite form of entender. In Brazilian Portuguese, the preterite is commonly used for a completed action in the past.

So entendi means:

• I understood
• or, in context, I did understand / I got it

With não, it becomes I didn’t understand.

These two forms express different ideas:

• entendi = I understood / I didn’t understand, referring to a specific completed moment
• entendia = I used to understand / I was understanding / I understood in an ongoing background sense, depending on context

In Eu não entendi esta parte da lição, the speaker means they failed to understand that part at a specific moment, so entendi is the natural choice.

Both esta and essa can appear in Brazilian Portuguese, and in everyday speech many Brazilians use essa more often than strict grammar rules would suggest.

Traditionally:

• esta = this, near the speaker
• essa = that, near the listener or previously mentioned

So esta parte literally means this part. In real Brazilian usage, essa parte da lição is also very common, especially in conversation.

Da is a contraction of de + a.

• de = of
• a lição = the lesson
• da lição = of the lesson

So esta parte da lição means this part of the lesson.

Portuguese often uses definite articles more frequently than English.

Here, a lição means the lesson, referring to a specific lesson already understood from context. Because of that, de + a becomes da.

• parte da lição = part of the lesson

Saying parte de lição would sound unnatural in this context.

No. Lição often means lesson, especially in educational contexts, but there are other possibilities depending on what you mean.

Common alternatives include:

• aula = class, lesson session
• capítulo = chapter
• material = material

For example:

• Não entendi esta parte da aula. = I didn’t understand this part of the class.
• Não entendi esta parte do material. = I didn’t understand this part of the material.

In Brazilian Portuguese, entendi is roughly pronounced like en-ten-DJI in many accents, though the final sound can vary a bit by region.

A helpful breakdown:

• en = like en in ten, but nasalized slightly
• ten = similar to English ten
• di = often sounds like djee in many parts of Brazil

So a rough approximation is en-ten-DJEE.

Lição is roughly pronounced lee-SOWN, with a nasal ending.

A rough breakdown:

• li = lee
• ção = something like sown, but nasal

The ção ending is very common in Portuguese. You will also see it in words like:

• informação
• situação
• nação

Yes. Compreender also means to understand, and não compreendi is correct.

However, entender is usually more common and more conversational in everyday Brazilian Portuguese.

So:

• Eu não entendi esta parte da lição. = very natural, everyday
• Eu não compreendi esta parte da lição. = also correct, slightly more formal or careful

Yes. In everyday Brazilian Portuguese, people often say:

• Não entendi essa parte da lição.
• Não entendi essa parte.
• Não entendi isso. if the context is clear

All of these can sound very natural, depending on the situation.

It is neutral and completely natural. It works well in both spoken and written Brazilian Portuguese.

It is not especially formal, but it is polite and clear. In casual conversation, people might shorten it to:

• Não entendi essa parte.
• Não entendi.

The full version is useful when you want to be specific.