Ele não abriu sequer uma mensagem.

Abriu is the simple past in Portuguese, also called the pretérito perfeito.

• abrir = to open
• ele abriu = he opened

In this sentence, it describes a completed action in the past: he did not open even a single message.

In standard Portuguese, não normally goes before the verb to make it negative:

• Ele não abriu = He did not open

Putting não after the verb can happen in spoken Brazilian Portuguese in some contexts, but it usually has a different tone or structure, for example:

• Ele abriu não sounds colloquial and marked, not the neutral standard form for this sentence.

So Ele não abriu... is the normal and correct choice here.

Sequer means something like:

• even
• even a single
• so much as

In this sentence, it strengthens the negative idea:

• Ele não abriu sequer uma mensagem.
• He didn’t open even one message.

It adds emphasis, showing that the amount was zero, not even the minimum possible.

Because they do different jobs.

• não makes the sentence negative.
• sequer adds emphasis: not even

Compare:

• Ele não abriu uma mensagem. = He did not open one message.
• Ele não abriu sequer uma mensagem. = He did not open even one message.

So sequer does not replace não here; it intensifies the idea.

Both are possible, but they are not exactly the same in tone.

• Ele não abriu nenhuma mensagem. = He didn’t open any messages.
• Ele não abriu sequer uma mensagem. = He didn’t open even one message.

The version with sequer uma is more emphatic. It highlights the idea that he didn’t open a single one.

So:

• nenhuma mensagem = neutral negative
• sequer uma mensagem = stronger, more emphatic negative

Because the phrase sequer uma mensagem focuses on one single message as the minimum unit.

It works like English:

• not even one message

So the singular is natural because uma is singular.

If you wanted a more general plural idea, you could say:

• Ele não abriu mensagens.
• Ele não abriu nenhuma mensagem.

But sequer uma mensagem specifically emphasizes not even one.

Yes, sometimes, but its position affects what it sounds like it is emphasizing.

In your sentence:

• Ele não abriu sequer uma mensagem.

This is very natural and clearly means not even one message.

You may also see sequer before the verb in other sentences, but in this structure it most naturally comes before the phrase it emphasizes:

• sequer uma mensagem

That is the cleanest and most idiomatic placement here.

Yes. Portuguese often drops subject pronouns when the meaning is clear from context.

So you could say:

• Não abriu sequer uma mensagem.

This is perfectly possible if the listener already knows who you are talking about.

However, ele may be included:

• for clarity
• for contrast
• for emphasis

For example, if you are comparing him with someone else, keeping ele makes sense.

In Brazilian Portuguese, abriu is usually pronounced in two syllables:

• a-briu

The ending -iu often forms one syllable in verbs like this.

So abriu sounds roughly like ah-BREEW, with the stress on the last syllable.

Yes, mensagem is a feminine noun, so it takes uma:

• a mensagem = the message
• uma mensagem = a message

If it were a masculine noun, you would use um instead.

For example:

• um recado = a message / note

So uma mensagem is simply the correct gender agreement.

Yes. Both capture the idea well, but they sound slightly different in English.

• He didn’t even open a message = natural, but can sound a little less explicit
• He didn’t open even one message = closer to the force of sequer uma mensagem

Because sequer strongly emphasizes the minimum amount, even one message is often the best match.

Because abriu presents the action as a completed event.

• não abriu = he did not open
• não abria = he was not opening / he wouldn’t open / he didn’t used to open, depending on context

So:

• Ele não abriu sequer uma mensagem. = a finished past fact
• Ele não abria sequer uma mensagem. = more like a habitual or ongoing past situation

In your sentence, the simple past is the natural choice if you are talking about one completed situation.