Eu não esperava isso.

Yes. In Brazilian Portuguese, subject pronouns are often optional, so Não esperava isso is a very natural sentence.

Eu is included for one of these reasons:

• to add emphasis
• to make the subject extra clear
• to contrast with someone else, as in Eu não esperava isso, mas ela sim

So both are correct:

• Eu não esperava isso
• Não esperava isso

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

So:

• Eu não esperava isso = I did not expect that

This is the standard pattern:

• Eu não sei
• Ela não gosta
• Nós não fomos

English uses do/did not, but Portuguese usually just puts não before the main verb.

Esperava is the imperfect past of esperar.

For esperar, the imperfect forms are:

• eu esperava
• você/ele/ela esperava
• nós esperávamos
• vocês/eles/elas esperavam

In this sentence, esperava gives a past meaning like:

• I wasn’t expecting that
• I didn’t expect that

Even though the imperfect often suggests an ongoing, habitual, or background past action, in real Portuguese this form is also very common for softening or framing expectations in the past.

Both can be grammatical, but they do not feel exactly the same.

• Eu não esperava isso often sounds like I wasn’t expecting that or That was not what I expected
• Eu não esperei isso is much less natural for this meaning

This is because esperar can mean both:

• to wait
• to expect

With esperei, many learners may accidentally drift toward the waited meaning:

• Eu esperei = I waited

For expressing a reaction like I didn’t expect that, Brazilian Portuguese very naturally uses:

• Não esperava isso
• Não esperava por essa
• Não imaginava isso depending on context

So esperava is the normal and idiomatic choice here.

Isso usually means that, especially something close to the listener or something just mentioned.

In modern Brazilian Portuguese:

• isto exists, but is much less common in everyday speech
• isso is very common and often replaces isto in casual use

Roughly speaking:

• isto = this
• isso = that
• aquilo = that over there / that thing

But in real Brazilian Portuguese conversation, isso is often used very broadly for that / this thing / what you said / what happened.

So Eu não esperava isso is completely natural.

Yes, but there is a nuance.

• Eu não esperava isso is the most direct and common way to say I didn’t expect that
• Eu não esperava por isso is also possible and can sound slightly more like I wasn’t expecting this to happen or I didn’t see this coming

In Brazilian Portuguese, both may be heard, but esperar algo without por is very common when esperar means to expect.

Compare:

• Eu espero uma resposta = I expect a response
• Eu estava esperando por você = I was waiting for you

So for your sentence, Eu não esperava isso is the most straightforward choice.

Yes, you can. Changing the word order can add emphasis.

Compare:

• Eu não esperava isso = neutral
• Isso eu não esperava = That I didn’t expect
• Isso não esperava = not standard in this context, because the subject is missing in a way that sounds awkward

So Isso eu não esperava is a very natural emphatic version, especially in speech.

In Brazilian Portuguese, esperava is pronounced approximately like:

es-pe-RA-va

A rough English-friendly guide:

• es like es in especially
• pe like peh
• RA is the stressed syllable
• va like vah

So the stress is on:

• es-pe-RA-va

And the full sentence:

• Eu não esperava isso

A rough approximation might be:

• eh-ooh nowng es-peh-RAH-vah EE-soo

But native pronunciation varies by region, especially for eu, não, and the final vowels.

Yes. Esperar can mean both to wait and to expect.

You understand the meaning from context.

Examples:

• Estou esperando o ônibus = I am waiting for the bus
• Espero boas notícias = I expect good news

In Eu não esperava isso, the meaning is clearly expect, not wait, because isso refers to some event, result, or statement.

So here:

• esperava = was expecting / expected

It is neutral and works in both formal and informal situations.

You could use it:

• in conversation
• in writing
• in a movie or TV show
• in a more serious context

If you want something more conversational, you might also hear:

• Não tava esperando isso = I wasn’t expecting that

That version is more informal because tava is a spoken contraction of estava.

Not naturally in this exact sentence.

Portuguese can use object pronouns, but with a demonstrative idea like that, isso is the normal choice.

So:

• Eu não esperava isso = natural
• Eu não o esperava usually means I wasn’t waiting for him/it, not I didn’t expect that

That is because o usually refers back to a masculine noun, while isso points to an idea, event, statement, or situation.

Yes, absolutely. It is a very normal sentence.

In everyday Brazilian Portuguese, you might hear:

• Eu não esperava isso
• Não esperava isso
• Não tava esperando isso
• Isso eu não esperava

They all sound natural, with small differences in tone, emphasis, or formality.