O mecânico vai balancear os pneus.

Vai balancear is the very common Brazilian Portuguese way to talk about the future: ir + infinitive.

• vai = goes / is going
• balancear = to balance

Together, vai balancear means is going to balance and often also works as will balance in natural English.

A simple future form is also possible:

• O mecânico balanceará os pneus.

That is correct, but it sounds more formal and is less common in everyday speech than vai balancear.

It can match both, depending on context.

In Brazilian Portuguese, vai + infinitive often covers the same idea that English expresses with either is going to or will. So this sentence could be understood as:

• The mechanic is going to balance the tires.
• The mechanic will balance the tires.

The Portuguese structure itself does not force a big difference between those two English options here.

Portuguese uses definite articles more often than English.

• O mecânico = the mechanic
• os pneus = the tires

In this sentence, os pneus normally refers to a specific set of tires, probably the tires of the car being worked on. In English, you might sometimes omit the in similar situations, but Portuguese usually keeps it.

If you said just pneus without os, it would sound more general or less anchored to a specific situation.

Pneu is a masculine noun:

• singular: o pneu
• plural: os pneus

That is why the article is os, not as.

Examples:

• o pneu = the tire
• os pneus = the tires

If it were singular in your sentence, it would be:

• O mecânico vai balancear o pneu.

Yes. In an automotive context, balancear os pneus is a normal and natural way to say to balance the tires.

This is specifically about tire balancing, not wheel alignment. That distinction matters:

• balancear os pneus = balance the tires
• alinhar or fazer alinhamento = align the wheels

So if a shop is correcting vibration caused by uneven weight distribution, balancear is the right word.

The accent in mecânico is part of the correct spelling and helps show the stressed syllable.

The stress falls on câ:

• me-CÂ-ni-co

Without the accent, the word would be misspelled. For learners, the key point is that the accent tells you where the emphasis goes and helps signal the vowel quality.

Yes, pneus is a word many English speakers find tricky.

Two important points:

• The p is pronounced.
• The stress is on pneu.

A helpful approximation is:

• pneu ≈ pnew
• pneus ≈ pnews

So unlike English words such as pneumatic, where the p is silent, Brazilian Portuguese normally pronounces it.

Yes, both are grammatical.

• O mecânico vai balancear os pneus.
  Most common and natural in everyday Brazilian Portuguese.

• O mecânico irá balancear os pneus.
  Correct, a bit more formal or written.

• O mecânico balanceará os pneus.
  Also correct, but more formal and less common in casual speech.

So if you are aiming for natural spoken Brazilian Portuguese, vai balancear is usually the best choice.

Here is the structure:

• O = definite article, the
• mecânico = noun, mechanic
• vai = third-person singular of ir, go
• balancear = infinitive verb, to balance
• os pneus = direct object, the tires

So the pattern is:

subject + ir in the present + infinitive + object

That pattern is one of the most useful future structures in Brazilian Portuguese.

This is the normal, neutral word order in Portuguese:

O mecânico vai balancear os pneus.

That is the standard subject + verb + object order.

You can change the order in special contexts for emphasis, but for learners, this version is the most natural and safest to use. For example, putting os pneus first would sound marked and not like the default way to say it.