Não quero ir sozinha pela avenida quando está escuro.

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

So:

• Não quero = I do not want
• Quero = I want

This is the standard way to make a sentence negative in Portuguese. Unlike English, Portuguese does not use a separate helping verb like do.

So:

• I do not want to go → Não quero ir

Portuguese often leaves out subject pronouns when the verb already makes the subject clear.

• (Eu) quero = I want
• (Eu) não quero = I do not want

Because quero clearly shows I, eu is optional here.

You could say:

• Eu não quero ir sozinha...

But it is very common, and natural, to say:

• Não quero ir sozinha...

After verbs like querer (to want), Portuguese usually uses an infinitive for the next action.

• quero ir = I want to go
• quero comer = I want to eat
• quero sair = I want to leave / go out

So quero ir works just like want to go in English.

Sozinho/sozinha agrees with the person being described.

• sozinho = used for a masculine speaker
• sozinha = used for a feminine speaker

So this sentence sounds like it is being said by a woman or girl:

• Não quero ir sozinha... = I don’t want to go alone...

If a man were speaking, it would be:

• Não quero ir sozinho...

Here, sozinha means alone, by myself, or on my own.

It describes the subject of the sentence, not the avenue.

So:

• ir sozinha = to go alone
• ficar sozinha = to stay alone

In this sentence, it means the speaker does not want to go there without company.

Pela is the contraction of por + a.

• por a avenida → pela avenida

Here, pela avenida suggests movement along, down, or through the avenue.

So:

• ir pela avenida = to go along/down the avenue

By contrast:

• na avenida = on / in the avenue
• ir na avenida can be heard in spoken Brazilian Portuguese, but ir pela avenida is more clearly about moving along that route.

So pela avenida fits well because the speaker is talking about going somewhere by way of the avenue.

Pela is just por + a combined.

Because avenida is feminine singular, it takes the article a:

• a avenida

When por comes before a, they contract:

• por + a = pela

Other common contractions:

• por + o = pelo
• por + as = pelas
• por + os = pelos

So pela avenida literally comes from por a avenida, but native speakers use the contracted form.

Portuguese usually uses estar for temporary conditions or states, including expressions about how things are at a certain moment.

• está escuro = it is dark
• está claro = it is light / bright

This refers to the current condition of the environment, so estar is the natural choice.

Using ser here would sound wrong in normal Portuguese.

Because escuro is not describing avenida here.

In quando está escuro, the idea is when it is dark. This is an impersonal expression: escuro refers to the general surroundings, not to a feminine noun.

So it stays in the default masculine singular form:

• Está escuro. = It’s dark.
• Está claro. = It’s bright.

If you were directly describing a feminine noun, then it would change:

• A rua está escura. = The street is dark.
• A avenida está escura. = The avenue is dark.

But that is not what is happening in this sentence.

Portuguese often uses impersonal expressions for weather, time, and environmental conditions.

So:

• Está escuro. = It is dark.
• Está frio. = It is cold.
• Está tarde. = It is late.

English needs the dummy subject it, but Portuguese does not. The subject is simply omitted.

That is why quando está escuro is a very natural way to say when it’s dark.

Yes. That would also be correct, but it changes the nuance.

• quando está escuro = when it’s dark / a general or habitual situation
• quando estiver escuro = when it gets dark or when it is dark in a future/less certain sense

Your original sentence sounds like a general statement:

• I don’t want to go alone down the avenue when it’s dark.

If you are talking about a future occasion, quando estiver escuro may sound more precise.

Yes, the word order is natural.

• Não quero ir sozinha pela avenida quando está escuro.

That sounds normal and clear.

You could also hear variations such as:

• Não quero ir pela avenida sozinha quando está escuro.
• Eu não quero ir sozinha pela avenida quando está escuro.

These are also understandable, but the original version is very natural.

Yes, in many contexts it gives that general idea.

Literally, it means when it is dark, but in natural English it may be translated as:

• when it’s dark
• after dark
• at night

The exact best translation depends on context, but the Portuguese expression itself focuses on darkness rather than the clock time.

Avenida often translates as avenue, but in real use it can refer to a broad urban road or major street.

So depending on context, it might feel like:

• avenue
• main avenue
• large street
• major road in a city

The core idea is a larger street, often wider or more important than an ordinary rua.

Yes, it is a natural sentence.

A Brazilian speaker might also choose slightly different wording depending on region and style, for example:

• Não quero andar sozinha pela avenida quando está escuro.
• Não quero passar sozinha pela avenida quando está escuro.
• Não quero ir sozinha pela avenida à noite.

But your original sentence is perfectly normal and understandable in Brazilian Portuguese.