Quando você chamar o técnico aqui, eu vou lhe mostrar o problema.

Because after quando referring to a future event, Portuguese usually uses the future subjunctive.

• quando você chamar = when you call
• quando você chama = usually when/whenever you call in a habitual or repeated sense

So this sentence is talking about a specific future moment, which is why chamar is used.

Because você takes third-person singular verb forms in Brazilian Portuguese.

So you get:

• você chama
• você chamou
• você chamar

Even though você means you, grammatically it behaves like he/she.

Not necessarily. Chamar can mean several things, including:

• to call
• to call for
• to summon
• to ask someone to come

In this sentence, because of o técnico aqui, the most natural idea is to get the technician to come here or to call the technician over here. It could involve a phone call, but the verb itself does not require that interpretation.

Aqui means here, and it goes with the idea of calling the technician here.

So the sense is:

• chamar o técnico aqui = call/get the technician here

It is not describing mostrar. The speaker is not saying I’ll show the problem here; they are saying when you bring/call the technician here, I’ll show him/the technician the problem—or, depending on context, I’ll show you the problem.

Because Quando você chamar o técnico aqui is an introductory subordinate clause, and the main clause comes after it:

• Quando você chamar o técnico aqui,
• eu vou lhe mostrar o problema.

This is very similar to English punctuation in a sentence like:

• When you call the technician here, I’ll show you the problem.

The comma helps separate the two parts.

Ir + infinitive is the most common way to express the future in everyday Brazilian Portuguese.

So:

• eu vou lhe mostrar = very natural in speech
• eu lhe mostrarei = correct, but more formal or literary

Both mean the same thing, but vou mostrar sounds much more conversational.

Lhe is an indirect object pronoun. Here it means to you.

The verb mostrar often works like this:

• mostrar algo a alguém = to show something to someone

So in the sentence:

• o problema = the thing being shown
• lhe = the person receiving the showing

In other words:

• eu vou lhe mostrar o problema = I’m going to show you the problem

Traditionally, lhe matches você, because você takes third-person grammar.

So, in more grammar-based terms:

• vou lhe mostrar fits você

However, in real spoken Brazilian Portuguese, many people say:

• vou te mostrar

even when they are using você. That is extremely common.

So the choices are roughly:

• vou lhe mostrar = correct, somewhat more formal or careful
• vou te mostrar = very common in everyday speech
• vou mostrar para você = also very natural and very clear

Yes. That is completely natural in Brazilian Portuguese.

These are all possible:

• eu vou lhe mostrar o problema
• eu vou te mostrar o problema
• eu vou mostrar o problema para você

The last one is often especially clear for learners, because it avoids pronoun placement issues.

Because with mostrar, the thing being shown is normally the direct object, and the person you show it to is the indirect object.

So:

• mostrar o problema = show the problem
• mostrar o problema a alguém / para alguém = show the problem to someone

In this sentence:

• o problema = direct object
• lhe = indirect object

Grammatically, yes, but in Brazilian Portuguese the most natural version is usually:

• vou lhe mostrar

Other placements are possible, but they sound more formal, more written, or less Brazilian in tone:

• vou mostrar-lhe = grammatical, but less common in everyday Brazilian speech
• lhe vou mostrar = much less natural in Brazil

So for modern spoken Brazilian Portuguese, vou lhe mostrar is the best model here.

O técnico suggests a specific technician, probably one already known in the situation.

• o técnico = the technician
• um técnico = a technician

So this sentence sounds like both speakers know which technician they mean, or at least the speaker is referring to the relevant technician for the problem.

In some contexts, yes, quando can mean when or whenever. But here, because of the future-subjunctive form chamar, it most naturally means a specific future when.

So the idea is:

• When you call/get the technician here, I’ll show you the problem

If the sentence were talking about a repeated situation, the structure would probably be different.