Quando a conexão cai, não dá para usar o wi-fi direito.

Yes, cai is the 3rd person singular of cair, which literally means to fall. But in Portuguese, just like the connection drops in English, a conexão cai is a very natural way to say that the connection goes down, stops working, or gets interrupted.

So here cai is not about physical falling. It is a common figurative use:

• A internet caiu. = The internet went down.
• A ligação caiu. = The call dropped.

Because a conexão is singular, the verb is cai.

Não dá para is a very common Brazilian Portuguese expression meaning something like:

• it’s not possible to
• you can’t really
• there’s no way to

So não dá para usar o wi-fi direito means that using the wi-fi properly is not really possible under those conditions.

The positive form is dá para, meaning:

• it’s possible to
• you can

Examples:

• Dá para ir a pé. = It’s possible to go on foot.
• Não dá para ouvir nada. = You can’t hear anything.

This structure is extremely common in everyday speech.

In this expression, dar is being used impersonally. That means there is no specific subject like he, she, or it.

So não dá para + infinitive works as a set expression:

• Não dá para esperar. = It’s not possible to wait.
• Não dá para entender. = You can’t understand / It’s impossible to understand.

You can think of it as similar to English it’s not possible to..., where it does not refer to a real thing.

Portuguese often uses articles more than English does. So o wi-fi is very natural.

In many cases, when talking about a specific service or thing in a general everyday way, Brazilian Portuguese still uses the article:

• usar o celular
• ligar a televisão
• acessar a internet
• usar o wi-fi

You may sometimes hear article omission in certain contexts, but usar o wi-fi sounds very normal.

Yes, direito can mean different things depending on context:

• right (opposite of left)
• law (the academic field or legal system)
• straight/upright
• and in colloquial use, properly, correctly, or well

In this sentence, usar o wi-fi direito means:

• use the wi-fi properly
• use the wi-fi correctly
• use the wi-fi well

This use of direito is very common in spoken Brazilian Portuguese.

Examples:

• Fala direito. = Speak properly / Speak clearly.
• Não consigo dormir direito. = I can’t sleep properly.
• Ela não explicou direito. = She didn’t explain it well.

Yes, but it would sound more formal and less natural in casual speech.

Compare:

• usar o wi-fi direito = everyday, natural, conversational
• usar o wi-fi corretamente = more formal, more careful, less colloquial

In normal conversation, Brazilians would usually prefer direito here.

Quando means when. It introduces a time clause:

• Quando a conexão cai = When the connection drops

This sets up the situation, and the second part gives the result:

• não dá para usar o wi-fi direito = it’s not possible to use the wi-fi properly

This structure is very common:

• Quando chove, eu fico em casa. = When it rains, I stay home.
• Quando ele chega tarde, jantamos sem ele. = When he arrives late, we have dinner without him.

The comma separates the introductory clause from the main clause:

• Quando a conexão cai, = introductory time clause
• não dá para usar o wi-fi direito. = main clause

This is standard punctuation and helps readability. In short spoken sentences, people may not think much about it, but in writing the comma is appropriate here.

Yes. Conexão is a broad word and can refer to different kinds of connection depending on context:

• internet connection
• network connection
• a transport connection
• even abstract connection in some contexts

In this sentence, because of usar o wi-fi, the meaning is clearly a network/internet connection.

Yes, wi-fi is commonly used and understood in Brazilian Portuguese. In informal writing, people may also write:

• wifi
• Wi-Fi

The meaning stays the same. In speech, pronunciation can vary a bit, but everyone will understand it.

Yes. The meaning would be very similar:

• Quando a conexão cai, não dá para usar o wi-fi direito.
• Quando a conexão cai, não é possível usar o wi-fi direito.

The difference is tone:

• não dá para = more natural and conversational
• não é possível = a bit more formal or neutral

In everyday Brazilian Portuguese, não dá para is extremely common.

Yes, very natural. It sounds like normal everyday spoken Brazilian Portuguese.

It combines several common features of casual, natural speech:

• a conexão cai
• não dá para
• usar o wi-fi
• direito meaning properly

A Brazilian speaker would easily say something like this in regular conversation.