Ontem à noite teve um temporal, e por isso a internet caiu.

In Brazilian Portuguese, ter is very commonly used in everyday speech with the meaning of there was / there were.

So:

• Teve um temporal = There was a storm
• More formal / traditional: Houve um temporal

A learner may expect haver here, because many textbooks teach há / houve for existence. That is still correct, especially in formal writing, but in Brazil ter is extremely common in conversation.

So this sentence sounds very natural in spoken Brazilian Portuguese.

Because ter here is being used impersonally, meaning it does not really have a normal subject.

In this use, Brazilian Portuguese normally keeps the verb in the 3rd person singular:

• Teve um temporal
• Teve muitos problemas
• Teve várias pessoas na festa

Even if what comes after is plural, the verb often stays singular in this colloquial existential use.

The more formal equivalent with haver also stays singular:

• Houve muitos problemas

So teve is not agreeing with um temporal as a subject in the normal way.

In this sentence, temporal means a heavy storm, often with strong rain, wind, thunder, or generally severe weather.

It does not mean temporal in the English sense of related to time.

In Brazilian Portuguese:

• um temporal = a storm / a bad storm / a downpour with stormy weather

It is a very common weather word.

À is the contraction of:

• a (preposition)
• a (feminine article)

So à noite is a fixed expression meaning at night / last night, depending on context.

Examples:

• Estudo à noite. = I study at night.
• Ontem à noite = last night

The grave accent marks crase, which shows that these two a sounds have merged into à.

This is why it is not just a noite here.

No. Ontem à noite is very common and neutral, but you may also hear:

• ontem de noite

In Brazil, both are used. In many contexts, à noite sounds a bit more standard or careful, while de noite is very common in speech.

So:

• Ontem à noite teve um temporal
• Ontem de noite teve um temporal

Both are natural in Brazilian Portuguese.

Por isso means because of that, for that reason, or so.

It connects the first idea to the result:

• teve um temporal = there was a storm
• por isso a internet caiu = because of that, the internet went down

It is a very common linking expression when one event causes another.

Similar expressions include:

• por causa disso = because of that
• então = so / then
• por esse motivo = for that reason

The comma helps separate the two clauses and makes the cause-result relationship clearer.

The sentence has two parts:

1. Ontem à noite teve um temporal
2. e por isso a internet caiu

In Portuguese, commas before e are usually avoided when the sentence is simple, but they are often used when:

• the clauses are longer,
• there is a shift in idea,
• or an inserted connector like por isso appears.

So this comma is natural and helps readability.

You may also see similar punctuation in real texts:

• Choveu muito, e por isso o jogo foi cancelado.

Portuguese often uses cair literally as to fall, but also figuratively for services or systems stopping.

So:

• a internet caiu = the internet went down
• a conexão caiu = the connection dropped
• o sistema caiu = the system crashed / went down
• a energia caiu is less common than acabou a energia, but similar ideas exist in technical contexts

This is a very normal way to talk about internet or system failure in Portuguese.

Portuguese often uses definite articles more than English does.

So where English says:

• the internet went down

Portuguese naturally says:

• a internet caiu

Even when speaking generally, Portuguese often includes the article with nouns:

• o Brasil
• a música
• a internet
• o João in many Brazilian dialects

So a internet is completely normal.

Both are in the pretérito perfeito (simple past / preterite).

They describe completed events in the past:

• teve = there was
• caiu = went down / fell

This tense is used because the sentence refers to specific finished events that happened yesterday night.

Compare:

• Ontem à noite teve um temporal. = specific completed event
• Quando eu era criança, sempre tinha tempestades no verão. = repeated or ongoing past situation

So the preterite is the natural choice here.

Sometimes, but the meaning and style change a little.

• Teve um temporal = very natural in spoken Brazilian Portuguese, a completed event
• Houve um temporal = more formal, also a completed event
• Havia um temporal = possible in some contexts, but sounds more literary or descriptive
• Tinha um temporal = heard in speech in some regions or informal contexts, but less standard for this exact sentence

Since the sentence is about a specific event that happened last night, teve fits very well.

That would sound less natural for this meaning.

Por isso usually introduces the result or consequence in a smoother way when placed before the clause:

• Teve um temporal, e por isso a internet caiu.

If you say:

• A internet caiu por isso

it can sound more like the internet went down because of that, but without the same natural flow as the original sentence.

Other natural alternatives would be:

• Ontem à noite teve um temporal, por isso a internet caiu.
• Ontem à noite teve um temporal, então a internet caiu.
• Como teve um temporal ontem à noite, a internet caiu.

Yes, in standard usage it is normally treated as feminine:

• a internet
• a internet caiu
• a internet está lenta

That is why the article is a, not o.

Yes, absolutely. It sounds very natural in Brazilian Portuguese.

Especially natural features are:

• teve used existentially
• um temporal for a bad storm
• a internet caiu for the internet going down

A Brazilian might also say similar versions like:

• Ontem à noite teve uma tempestade, então a internet caiu.
• Teve um temporal ontem à noite, e a internet caiu.
• Por causa do temporal, a internet caiu.

But the original sentence is perfectly normal and idiomatic.