Eu vou pagar pela passagem hoje à tarde.

Vou pagar is the very common everyday way to say I am going to pay / I will pay in Brazilian Portuguese.

It uses:

• vou = I go / I am going
• pagar = to pay

So literally it is I am going to pay, but in real use it often simply means I will pay.

The simple future pagarei is correct, but it sounds more formal, more written, or sometimes more emphatic. In normal conversation, Brazilians very often prefer vou + infinitive.

Examples:

• Eu vou ligar amanhã. = I’ll call tomorrow.
• Eu ligarei amanhã. = also correct, but more formal or less conversational

Yes, you can usually omit Eu.

Portuguese often drops subject pronouns when the verb already makes the subject clear. Since vou clearly means I go / I am going, Eu is optional.

So both are natural:

• Eu vou pagar pela passagem hoje à tarde.
• Vou pagar pela passagem hoje à tarde.

Including Eu can add emphasis, contrast, or clarity.

For example:

• Eu vou pagar, não ele. = I’m going to pay, not him.

Pela is a contraction of por + a.

• por can mean for, through, by, depending on context
• a is the feminine singular definite article the
• por + a = pela

With pagar, Portuguese often uses pagar por for pay for something.

So:

• pagar pela passagem = pay for the ticket / fare

You may also hear pagar a passagem, especially when the thing being paid is treated more directly as the object. But pagar pela passagem is completely natural and very common.

Compare:

• Vou pagar pela comida. = I’m going to pay for the food.
• Vou pagar a conta. = I’m going to pay the bill.

Some nouns are very commonly used directly after pagar, while in other cases pagar por/pela/pelo is very natural.

Passagem can mean several related things depending on context:

• ticket
• fare
• passage
• sometimes even trip segment or travel ticket

In Brazilian Portuguese, passagem is very commonly used for transportation tickets:

• passagem de ônibus = bus ticket / bus fare
• passagem de avião = plane ticket
• passagem de trem = train ticket

So in this sentence, passagem most likely means the ticket or the fare, depending on the situation.

Because à here is a contraction of a + a.

This happens because the expression uses:

• the preposition a
• plus the feminine article a before tarde

So:

• a + a = à

In time expressions, this is very common:

• à tarde = in the afternoon
• à noite = at night / in the evening
• às duas horas = at two o’clock

The grave accent marks crase, which shows that two a sounds/words have combined into one.

• à tarde = in the afternoon
• hoje à tarde = this afternoon / today in the afternoon

So hoje makes the time more specific.

Examples:

• Vou estudar à tarde. = I study / will study in the afternoon (in general or on some relevant day)
• Vou estudar hoje à tarde. = I’m going to study this afternoon (specifically today)

Yes. That is completely natural.

Portuguese allows some flexibility in word order, especially with time expressions. These all sound normal:

• Eu vou pagar pela passagem hoje à tarde.
• Hoje à tarde eu vou pagar pela passagem.
• Hoje à tarde vou pagar pela passagem.

The difference is mainly emphasis:

• ending with hoje à tarde gives the time as the final piece of information
• starting with Hoje à tarde emphasizes when

In many Brazilian accents, the final r in infinitives is softer than English speakers expect, and in casual speech it may sound weak or almost disappear.

So pagar is often pronounced roughly like:

• pah-GAR in careful speech
• or something closer to pah-GAH in more casual pronunciation, depending on the region

A few pronunciation notes:

• g in pagar is a hard g, like in go
• the stress is on the last syllable: pa-GAR

Regional pronunciation of final r varies a lot in Brazil, so don’t be surprised if you hear different versions.

They express different ideas:

• vou pagar = I’m going to pay / I will pay
  This usually talks about a future action.

• estou pagando = I am paying
  This describes an action in progress right now, or sometimes a temporary ongoing situation.

So:

• Eu vou pagar pela passagem hoje à tarde. = the payment will happen later today
• Eu estou pagando pela passagem agora. = I’m paying for the ticket right now

It would be understood throughout the Portuguese-speaking world, and the grammar is standard.

That said, a few things are especially common in Brazilian usage:

• frequent use of vou + infinitive for the future
• everyday use of passagem for transportation ticket/fare

In European Portuguese, the sentence is also correct and understandable, though pronunciation and some usage patterns may differ.