El wifi dejó de funcionar justo cuando iba a empezar la videollamada.

In Spanish, nouns are normally used with an article more often than in English, especially when you are talking about a specific thing in a situation.

So el wifi here means the Wi-Fi / the internet connection.

A few notes:

• wifi is treated as masculine in standard usage in Spain, so el wifi is the usual form.
• In casual speech, people do sometimes say just wifi, but el wifi sounds very natural in a full sentence like this.
• You may also hear la wifi in some places, but el wifi is generally the safer choice.

Literally, dejó de funcionar means it stopped functioning or it ceased to work.

This uses the pattern:

• dejar de + infinitive = to stop doing something

So:

• dejar de fumar = to stop smoking
• dejar de hablar = to stop talking
• dejar de funcionar = to stop working / stop functioning

In this sentence, it is a very natural way to say that the Wi-Fi stopped working.

Dejó is in the preterite because the sentence is talking about a completed event: at a certain moment, the Wi-Fi stopped working.

The preterite is commonly used for:

• single completed actions
• events that happen at a definite point in time
• actions that move the story forward

Here, the important event is:

• the Wi-Fi stopped

That is why dejó works well.

If you used the imperfect instead, such as dejaba, it would sound like background information or a repeated/habitual action, which does not fit this context well.

Iba a empezar means was going to start or was about to start.

This is the structure:

• ir a + infinitive

In the imperfect:

• iba a empezar = I was going to start / I was about to start

It is used here because the action had not fully happened yet. The speaker was on the point of starting the video call when the Wi-Fi failed.

Compare:

• iba a empezar = was about to start
• empecé = I started
• empezaba = I was starting / used to start, depending on context

So iba a empezar is the best choice if the call had not actually begun yet.

The subject is probably yo, but Spanish often leaves subject pronouns out because the verb form and context usually make the meaning clear.

So:

• yo iba a empezar la videollamada
  can simply become
• iba a empezar la videollamada

This is very normal in Spanish.

One useful detail: iba could also technically mean he/she/it was going to, because the imperfect forms for yo and él/ella/usted are the same. The context tells you who the subject is.

Yes, you could say just cuando, but justo cuando adds emphasis.

• cuando = when
• justo cuando = just when / right when

So justo cuando highlights the bad timing:

• the Wi-Fi stopped working right at the moment the video call was about to begin

It makes the sentence sound a bit more expressive and natural.

That a is part of the fixed future/intention structure:

• ir a + infinitive

Examples:

• voy a comer = I am going to eat
• íbamos a salir = we were going to leave
• iba a empezar = I was going to start

So the a does not mean to in the same way as in English word-for-word. It is simply a required part of this construction.

For the same general reason as el wifi: Spanish often uses articles where English might not.

Here la videollamada means:

• the video call

It refers to a specific call that both speaker and listener can identify from the context.

Also:

• videollamada is feminine, so it takes la
• singular: la videollamada
• plural: las videollamadas

Yes. Videollamada is normally written as one word in modern standard Spanish.

It combines:

• video
• llamada

So:

• videollamada = video call

This is the standard spelling you should learn and use.

Yes, in many contexts you could, and it sounds very natural in everyday Spanish.

Compare:

• el wifi dejó de funcionar = the Wi-Fi stopped working
• se cayó el wifi = the Wi-Fi went down / the Wi-Fi dropped

The difference is mostly stylistic:

• dejó de funcionar is a bit more neutral and literal
• se cayó is very common in colloquial speech when a connection fails

Both are useful, but dejó de funcionar is very clear and broadly understood.

Yes. That is another very natural option.

• iba a empezar la videollamada = I was going to start the video call
• estaba a punto de empezar la videollamada = I was just about to start the video call

Estar a punto de + infinitive puts even more focus on how immediate the action was.

So both work, but:

• iba a empezar = more general was going to start
• estaba a punto de empezar = more specifically was just about to start