A campainha tocou quando eu estava saindo para o trabalho.

Because Brazilian Portuguese is contrasting a completed event with an ongoing action.

• A campainha tocou = The doorbell rang / rang once / at that moment
• eu estava saindo = I was leaving / I was in the process of leaving

So tocou is in the preterite, which is commonly used for a finished event that happened at a specific moment.
If you said a campainha tocava, it would usually suggest a repeated or ongoing ringing in the past, which changes the image.

Both can refer to a past ongoing action, but they are not exactly the same in feel.

• eu estava saindo = I was leaving / emphasizes the action in progress at that moment
• eu saía = can mean I used to leave or sometimes I was leaving, depending on context

In this sentence, estava saindo is very natural because it clearly highlights that the person was in the middle of leaving when the bell rang.

So the structure:

• estar
  • gerund
• estava saindo

is the usual way in Brazilian Portuguese to express something like was leaving, was doing, was eating, etc.

Saindo is the gerund of sair (to leave, to go out).

So:

• sair = to leave
• saindo = leaving

In eu estava saindo, it means I was leaving or I was on my way out.

Yes, it can be omitted.

Portuguese often leaves out subject pronouns when the verb already makes the subject clear:

• quando eu estava saindo
• quando estava saindo

Both are possible.

However, speakers often include eu for:

• clarity
• emphasis
• natural rhythm

In Brazilian Portuguese especially, subject pronouns are used more often than in some other varieties of Portuguese, so eu estava saindo sounds completely normal.

A is the feminine singular definite article, meaning the.

• a campainha = the doorbell
• uma campainha = a doorbell

The noun campainha is feminine, so it takes a.

That is exactly how the word is built.

• campana/campa historically relates to bell
• -inha is a diminutive ending, meaning something like small or little

So campainha originally suggests a little bell, and in modern usage it commonly means doorbell or bell depending on context.

In this sentence, A campainha tocou is naturally understood as The doorbell rang.

Yes. Tocar has several meanings, and this is very common in Portuguese.

It can mean:

• to touch
• to play an instrument
• to ring / to sound (for bells, alarms, phones, etc.)

So in this sentence:

• A campainha tocou = The doorbell rang

This is a normal and everyday use of tocar.

Both can exist, but they do not always sound the same.

• para o trabalho = for work / to work
• ao trabalho = literally to the workplace/work

In Brazilian Portuguese, sair para o trabalho is a very natural expression meaning to leave for work.

It focuses on your purpose or destination in a broad sense: you are heading out because you are going to work.

By contrast, ir ao trabalho can also be used, but it sounds a little more like going to the workplace itself.

Portuguese uses definite articles more often than English does.

Here:

• o trabalho = literally the work

But in natural English we usually just say work.

So para o trabalho is the normal Portuguese way to express to work in this context.

Quando means when and introduces the time clause.

The structure is:

• A campainha tocou = main event
• quando eu estava saindo para o trabalho = what was happening at that time

So the sentence shows that the bell rang at the moment when the speaker was leaving for work.

Yes.

You can also say:

• Quando eu estava saindo para o trabalho, a campainha tocou.

This means the same thing. The difference is mostly one of focus and style.

• Starting with A campainha tocou puts the bell-ringing first
• Starting with Quando eu estava saindo... sets the scene first

Both are natural.

In Brazilian Portuguese, saindo is pronounced roughly like sah-EEN-doo, but that is only an approximation.

A few points:

• sa- sounds like sa
• -in- here is part of indo, not like English in
• -do is an unstressed final syllable, often sounding like doo in many Brazilian accents

Also, sair and its forms can be a little tricky because the vowels are pronounced separately:
sa-ir, sa-in-do

Approximately: kahm-pah-EEN-yah

Important parts:

• cam- has a nasal quality in Portuguese
• pai here sounds roughly like pah-ee
• nh is a special Portuguese sound, similar to the ny in canyon

So campainha ends with a sound like -eenya.

Yes, soou is possible because soar means to sound.

• A campainha tocou = the most everyday, common way to say The doorbell rang
• A campainha soou = also understandable, but usually less common in casual speech

For a learner, tocou is the safest and most natural choice in everyday Brazilian Portuguese.

By itself, campainha often suggests a doorbell, especially in everyday conversation.

But depending on context, it could also refer to another small bell or buzzer.
Still, if someone says:

• A campainha tocou

most people will naturally think of a doorbell unless the situation suggests something else.