A conexão do wi-fi está fraca hoje, então o vídeo demora para abrir.

Because do is the contraction of de + o.

In Brazilian Portuguese, wi-fi is commonly treated as a masculine noun: o wi-fi. So:

• de + o wi-fi → do wi-fi

Literally, a conexão do wi-fi is something like the connection of the Wi‑Fi, but in natural English we usually say the Wi‑Fi connection.

A useful comparison:

• a conexão da internet = the internet connection
• a conexão do wi-fi = the Wi‑Fi connection

Here, da is used with internet because internet is feminine: a internet.

Because adjectives in Portuguese usually agree with the noun they describe.

The noun here is conexão, which is feminine singular:

• a conexão

So the adjective must also be feminine singular:

• fraca

That is why you get:

• A conexão ... está fraca

If the noun were masculine, you would use fraco:

• O wi-fi está fraco

Because estar is used for a temporary condition or state, while ser is used for more permanent characteristics, identity, classification, and similar ideas.

Here, the connection is weak today, so this is a temporary situation:

• está fraca hoje

That makes estar the natural choice.

Compare:

• A conexão está fraca hoje = The connection is weak today
• Ela é fraca would sound more like an inherent characteristic, which does not fit well here

Here, então means so, therefore, or as a result.

It connects the cause and the result:

• A conexão do wi-fi está fraca hoje = cause
• então o vídeo demora para abrir = result

So the logic is:

The Wi‑Fi connection is weak today, so the video takes a while to open.

Be aware that então can also mean then in other contexts, depending on the sentence.

Because então is linking two clauses, and in writing it is very common to put a comma before it when it means so/therefore.

So this punctuation helps show the pause and the cause-result relationship:

• A conexão do wi-fi está fraca hoje, então o vídeo demora para abrir.

In speech, you would normally hear a small pause there too.

The verb demorar means to take a long time or to be slow.

So:

• demora para abrir = takes a long time to open

Very literally, it is something like:

• the video delays/is slow to open

But in natural English, you would usually say:

• the video takes a while to open
• the video is slow to open
• the video takes a long time to load/open

Also, in tech contexts, abrir can mean not just open, but also start up, load, or come up on the screen, depending on context.

Because in Brazilian Portuguese, demorar is very commonly followed by para + infinitive when you say that something takes time to happen.

Pattern:

• demorar para + verb

Examples:

• O vídeo demora para abrir.
• Ela demorou para responder.
• O programa demora para carregar.

So para is the normal link before the infinitive here.

You may also see demorar a + infinitive, especially in more formal language or in European Portuguese, but in Brazil demorar para is very common and natural.

Because Portuguese uses articles much more often than English does.

With a singular countable noun like vídeo, Portuguese normally wants an article unless there is another determiner.

So:

• o vídeo = the video
• um vídeo = a video

In this sentence, o vídeo sounds natural because the speaker is referring to the video in question, the one being watched or discussed.

Using no article here would sound incomplete or unnatural in standard Portuguese.

Yes, often you can, but the nuance changes a little.

• abrir = to open, start, come up
• carregar = to load

So:

• O vídeo demora para abrir suggests the video is slow to open/start
• O vídeo demora para carregar emphasizes the loading process more specifically

In internet and app contexts, carregar is very common and often sounds especially natural when the problem is slow connection.

Because adverb placement in Portuguese is somewhat flexible, and this position is very natural.

• A conexão do wi-fi está fraca hoje.

Here, hoje modifies the whole situation and sounds smooth at the end of that clause.

You could also say:

• Hoje, a conexão do wi-fi está fraca.
• A conexão do wi-fi hoje está fraca.

These are possible, but they shift the emphasis a bit:

• Hoje, ... puts more emphasis on today
• ... está fraca hoje sounds more neutral and conversational

Yes, absolutely. That is a very natural alternative.

• O wi-fi está fraco hoje = The Wi‑Fi is weak today
• A conexão do wi-fi está fraca hoje = The Wi‑Fi connection is weak today

The second version is a bit more explicit because it mentions the connection itself. The first version is shorter and very common in everyday speech.

Notice the adjective change:

• o wi-fi → fraco
• a conexão → fraca

The accent mark shows the stressed syllable.

• ví-de-o

The stress falls on the first syllable: ví

Without going too deep into spelling rules, the accent is there because the written form needs to show that this word does not follow the most basic default stress pattern.

Accent marks in Portuguese are very important because they help with both pronunciation and sometimes meaning.

Yes, it is natural and understandable.

That said, Brazilians might also say similar things in slightly different ways, such as:

• A internet está lenta hoje, então o vídeo demora para carregar.
• O wi-fi está ruim hoje, então o vídeo não abre direito.
• A conexão está fraca hoje, então o vídeo demora para abrir.

The original sentence is perfectly fine. Other versions may sound more casual or more specific depending on whether the speaker wants to emphasize weak signal, slow internet, or slow loading.