Se algo estranho acontecer de novo, me avise imediatamente.

After se in a real future possibility, Portuguese usually uses the future subjunctive.

So in Se algo estranho acontecer de novo..., acontecer is not just the infinitive in meaning; it is the future subjunctive form.

This is very common in Portuguese:

• Se ele vier, eu aviso.
• Se chover, ficamos em casa.
• Se algo estranho acontecer, me avise.

For many verbs, the future subjunctive looks exactly like the infinitive in some forms:

• acontecer → future subjunctive singular: acontecer
• fazer → fizer
• ir → for

So an English speaker may think acontecer is just the infinitive here, but grammatically it is the future subjunctive.

Me avise is the standard imperative/command form for você.

In Brazilian Portuguese, commands for você are formed with the present subjunctive:

• avisar → avise
• falar → fale
• abrir → abra

So:

• Me avise imediatamente. = standard way to say Let me know immediately

You will also hear me avisa in casual spoken Brazilian Portuguese. That is very common in conversation, but me avise is the more standard and grammatically expected form in this sentence.

Yes. Avise shows that the speaker is addressing você.

In Brazil, você is extremely common, so this structure is very natural.

Compare:

• Me avise → command to você
• Me avisem → command to vocês
• Avisa-me or avisa → associated with tu, but usage varies a lot by region

Even though você is not written, the verb form tells you who is being addressed.

Yes, avise-me is grammatically correct, but in Brazilian Portuguese me avise is much more natural in everyday speech.

This is about clitic pronoun placement:

• me avise = very common in Brazil
• avise-me = correct, but more formal or more typical of careful writing / European Portuguese style

So if you are learning Brazilian Portuguese for real-life use, me avise is the form you will probably hear and use most.

Yes. De novo means again.

In this sentence, acontecer de novo means to happen again.

Very common alternatives are:

• de novo → very common, conversational
• novamente → a bit more formal or neutral
• outra vez → also common

So these are all possible:

• Se algo estranho acontecer de novo...
• Se algo estranho acontecer novamente...
• Se algo estranho acontecer outra vez...

In everyday Brazilian Portuguese, de novo is especially common.

In Portuguese, adjectives usually come after the noun or pronoun they describe.

So:

• algo estranho = something strange
• uma coisa estranha = a strange thing

With algo, nada, algo muito estranho, and similar expressions, the adjective normally follows:

• algo importante
• nada sério
• algo inesperado

Putting estranho before algo would sound unnatural here.

Yes. That is also correct.

These two versions are both natural:

• Se algo estranho acontecer de novo, me avise imediatamente.
• Se acontecer algo estranho de novo, me avise imediatamente.

The difference is mostly one of word order and focus:

• Se algo estranho acontecer... puts algo estranho early, so the strange thing is highlighted a bit more.
• Se acontecer algo estranho... starts with the verb, which can sound slightly more neutral or event-focused.

Both are good Portuguese.

Because the sentence starts with a conditional clause:

• Se algo estranho acontecer de novo = the condition
• me avise imediatamente = the main clause

In Portuguese, when a subordinate clause like this comes first, it is normally separated by a comma.

So the comma helps show the structure:

• If X happens, do Y

This is standard punctuation:

• Se ele ligar, me avise.
• Quando você chegar, me chama.
• Caso precise de ajuda, me avise.

Yes, in many contexts algo and alguma coisa can both mean something.

Here:

• algo estranho = something strange
• alguma coisa estranha = also something strange

Difference in style:

• algo sounds a bit shorter and sometimes slightly more concise or written
• alguma coisa is also very common and natural in speech

So you could also say:

• Se alguma coisa estranha acontecer de novo, me avise imediatamente.

That said, algo estranho sounds perfectly natural.

Imediatamente means immediately, and it emphasizes urgency.

In me avise imediatamente, it tells the person to inform you right away, not later.

Its position is fairly flexible, but some placements sound more natural than others:

• Me avise imediatamente. → most natural here
• Me avise de imediato. → also natural
• Imediatamente, me avise. → possible, but more dramatic or marked

So the original position is the most normal one for everyday use.

It is neutral and natural Brazilian Portuguese.

Why?

• me avise is standard and polite without sounding stiff
• de novo is common and conversational
• the whole sentence sounds like normal spoken or written Brazilian Portuguese

It is not overly formal, but it is also not slangy.

If you wanted something more formal, you might say:

• Caso algo estranho aconteça novamente, por favor, avise-me imediatamente.

If you wanted something more casual, you might hear:

• Se acontecer alguma coisa estranha de novo, me avisa na hora.

So the original sentence sits comfortably in the middle.

Yes. Estranho can cover several nearby ideas depending on context:

• strange
• weird
• odd
• unusual
• sometimes even suspicious

So algo estranho is a flexible expression. It could refer to:

• weird behavior
• an unusual event
• something suspicious
• something that feels wrong or out of place

The exact nuance depends on the situation, but grammatically the phrase is straightforward.