Se o técnico aparecer hoje, eu vou avisar você imediatamente.

Because after se meaning if, Portuguese normally uses the future subjunctive when the condition refers to the future.

So:

• Se o técnico aparecer hoje... = If the technician shows up today...

This is the normal structure for a real future possibility in Portuguese.

Compare:

• Se o técnico aparece hoje... → not the standard choice for this future meaning
• Se o técnico aparecerá hoje... → incorrect here

A very common pattern is:

• se + future subjunctive, then
• a main clause with future meaning

Example:

• Se chover, eu fico em casa.
• Se ele ligar, eu te conto.

In this sentence, aparecer is the future subjunctive form of the verb aparecer.

For many verbs, this form looks identical to the infinitive in the singular:

• quando eu aparecer
• se ele aparecer

That can be confusing for English speakers, because it looks like the dictionary form, but grammatically it is functioning as a tense/mood form.

With other subjects, you can see the pattern more clearly:

• se eu aparecer
• se você aparecer
• se nós aparecermos
• se eles aparecerem

Portuguese uses this form after words like:

• se = if
• quando = when
• assim que = as soon as
• logo que = as soon as

when the action is in the future.

Vou avisar is the very common near future / periphrastic future in Brazilian Portuguese:

• eu vou avisar você = I’m going to let you know / I’ll let you know

It is extremely natural in everyday speech.

You could also say:

• eu avisarei você imediatamente

That is grammatically correct, but it sounds more formal or written. In spoken Brazilian Portuguese, vou + infinitive is usually more common.

So:

• vou avisar = natural, everyday
• avisarei = correct, more formal/literary

Yes. Portuguese often drops subject pronouns when the verb already shows who the subject is.

So all of these are possible:

• Se o técnico aparecer hoje, eu vou avisar você imediatamente.
• Se o técnico aparecer hoje, vou avisar você imediatamente.

Both are correct.

Including eu can add:

• emphasis
• clarity
• contrast

For example, if you want to stress that I will do it, not someone else, keeping eu makes sense.

Yes, and in everyday Brazilian Portuguese, te avisar is often more natural.

So these are both correct:

• vou avisar você imediatamente
• vou te avisar imediatamente

Difference in feel:

• avisar você can sound a bit more explicit or emphatic
• te avisar is very common and conversational

A native speaker might often prefer:

• Se o técnico aparecer hoje, vou te avisar imediatamente.

If you are speaking casually in Brazil, that version may sound more natural.

Avisar usually means:

• to let someone know
• to notify
• to warn, depending on context

In this sentence, the best sense is:

• to let you know
• to notify you

So vou avisar você imediatamente is something like:

• I’ll let you know right away
• I’ll notify you immediately

It does not necessarily sound as formal as notify in English; avisar is a very common everyday verb.

Portuguese uses definite articles more often than English does.

So o técnico means:

• the technician

If both speakers already know which technician they mean, the article is completely natural.

In Portuguese, leaving out the article would usually sound incomplete here. You would not normally say just:

• Se técnico aparecer hoje...

That is incorrect in this context.

So o técnico is simply the normal way to refer to a specific technician.

Here, técnico most likely means technician, such as:

• a repair technician
• an internet technician
• an appliance technician
• a cable technician

The exact meaning depends on context.

Be careful: técnico can also mean other things in different contexts, such as:

• coach in sports
• technical as an adjective in some expressions

But in this sentence, with aparecer hoje, the most natural interpretation is a service or repair technician who may show up.

Because the sentence begins with a subordinate if-clause:

• Se o técnico aparecer hoje, ...

When that dependent clause comes first, Portuguese normally separates it from the main clause with a comma.

So this punctuation is standard and natural:

• Se o técnico aparecer hoje, eu vou avisar você imediatamente.

If the main clause comes first, the comma is often not used:

• Eu vou avisar você imediatamente se o técnico aparecer hoje.

Both word orders are fine.

Yes. Adverbs in Portuguese often have some flexibility.

These are all possible:

• Se o técnico aparecer hoje, eu vou avisar você imediatamente.
• Se o técnico aparecer hoje, eu vou imediatamente avisar você.
• Se o técnico aparecer hoje, imediatamente eu vou avisar você.

The original version is the most neutral and natural.

For everyday speech, you might also hear:

• Se o técnico aparecer hoje, vou te avisar na mesma hora.
• Se o técnico aparecer hoje, vou te avisar imediatamente.

So imediatamente is correct, clear, and fairly neutral in tone.

Yes. Even if the meaning has already been given, that is a very natural English rendering.

Word-for-word, it is close to:

• If the technician appears today, I am going to notify you immediately.

But more natural English would usually be:

• If the technician shows up today, I’ll let you know right away.

That is a good reminder that aparecer often corresponds to:

• appear
• show up
• turn up

depending on context.