O poste elétrico caiu na rua.

O means the, while um means a/an.

So:

• o poste elétrico = the electric pole
• um poste elétrico = an electric pole

In this sentence, the speaker is referring to a specific pole, or presenting it as a definite object in the scene.

In Portuguese, descriptive adjectives often come after the noun.

So:

• poste elétrico = literally pole electric
• natural English = electric pole

This noun + adjective order is very common in Portuguese. Putting elétrico before the noun would sound unusual here.

Because poste is a masculine singular noun, so the adjective must agree with it.

That is why you get:

• o poste elétrico = masculine singular

If the noun were feminine, the adjective would change:

• a instalação elétrica = the electrical installation

Agreement is very important in Portuguese: articles and adjectives usually match the noun in gender and number.

Caiu is the 3rd person singular preterite of the verb cair, which means to fall.

So:

• cair = to fall
• caiu = fell

It is used for a completed action in the past. Here, it tells you that the pole fell.

Because Portuguese cair already contains that idea.

In English, we often say fall or fall down. In Portuguese, just cair is enough in most cases.

So:

• O poste elétrico caiu = The electric pole fell / fell down

You do not need an extra word for down here.

There is no it in this sentence because the subject is already named: O poste elétrico.

Portuguese also often leaves out subject pronouns when they are unnecessary. English usually needs a pronoun, but Portuguese often does not.

For example:

• Caiu na rua. = It fell in/into the street.
  This can work if the subject is already clear from context.

But in your sentence, the full subject is stated, so no pronoun is needed.

Na is a contraction of:

• em = in / on / at
• a = the (feminine singular)

So:

• em + a = na

Because rua is feminine, Portuguese uses a rua, and after contraction you get na rua.

This kind of contraction is very common:

• no = em + o
• na = em + a
• nos = em + os
• nas = em + as

Depending on context, it can match different English choices.

In this sentence, na rua can be understood as:

• in the street
• on the street
• into the street

English chooses different prepositions depending on how the scene is imagined. Portuguese na rua is broader here. With caiu, English often prefers into the street or in the street, depending on the exact image.

The normal, neutral order is:

O poste elétrico caiu na rua.
Subject + verb + place

That is the most natural order for a basic statement.

You can change the order for emphasis, but it becomes more marked:

• Na rua, o poste elétrico caiu.
• Caiu na rua o poste elétrico.

These are possible, but the original sentence is the most straightforward and natural for everyday use.