Eu descobri que posso gastar menos quando faço uma lista antes de comprar.

Yes. In Brazilian Portuguese, eu is often optional because the verb form already shows the subject.

So:

• Eu descobri...
• Descobri...

both mean the same thing here: I discovered / I found out...

Including eu can add emphasis, contrast, or clarity. For example, if you want to stress that I discovered it, not someone else, eu sounds more natural.

Descobri is the preterite form of descobrir.

• descobrir = to discover, to find out, to realize
• descobri = I discovered / I found out / I realized

In this sentence, descobri does not necessarily mean a dramatic discovery. Very often it simply means I realized or I found out.

Examples:

• Descobri que ele mora aqui. = I found out that he lives here.
• Descobri que estudar cedo funciona melhor para mim. = I realized that studying early works better for me.

Here, que means that and introduces the idea that was discovered.

• Descobri que... = I discovered that...
• Percebi que... = I noticed that...
• Sei que... = I know that...

In English, that is often optional, but in Portuguese que is usually kept in this kind of sentence.

So:

• Eu descobri que posso gastar menos...

is the normal structure.

Because the speaker discovered a general present ability or possibility.

• descobri = the moment of realization happened in the past
• posso gastar menos = what is true now or in general is expressed in the present

So the sentence means something like:

• I realized that I can spend less...

not necessarily

• I realized that I could spend less in a purely past-only sense

Portuguese often mixes a past main verb with a present verb in the subordinate clause when the second idea is still true now.

Compare:

• Descobri que posso correr 5 km. = I found out that I can run 5 km.
• Descobri que podia correr 5 km. = I found out that I could run 5 km.
  This can sound more tied to a past situation.

Gastar menos literally means to spend less.

• gastar = to spend
• menos = less

So posso gastar menos = I can spend less

This is related to economizar, but they are not always identical:

• gastar menos focuses on reducing spending
• economizar means to save, conserve, or economize

In this sentence, gastar menos is very natural because the idea is specifically about spending less money when shopping.

Examples:

• Quero gastar menos este mês. = I want to spend less this month.
• Quero economizar dinheiro este mês. = I want to save money this month.

Because quando faço uma lista antes de comprar describes a habitual or repeated situation, not one single completed event in the past.

The sentence means:

• I discovered that I can spend less when I make a list before buying.

Here quando faço is basically when(ever) I make.

Portuguese often uses the present tense for repeated truths or habits, even if the main verb is in the past.

Compare:

• Descobri que durmo melhor quando leio antes de dormir.
  = I discovered that I sleep better when I read before sleeping.

It is a general pattern, so present tense makes sense.

Faço is the eu form of fazer in the present tense, and fazer is irregular.

Present tense of fazer:

• eu faço
• você / ele / ela faz
• nós fazemos
• vocês / eles / elas fazem

So:

• quando faço uma lista = when I make a list

This is just one of the very common irregular verbs you will need to memorize.

Also notice the spelling:

• faço has ç so that it keeps the s sound before o

Without the cedilla, c before o would sound different.

Yes, most likely. Uma lista literally means a list, and in this context it naturally means a list of things to buy.

So:

• faço uma lista antes de comprar
  = I make a list before buying / before shopping

Even though Portuguese does not explicitly say shopping list, the meaning is clear from context.

You could also say:

• uma lista de compras = a shopping list

That version is more explicit.

Because Portuguese commonly uses antes de + infinitive when the subject is understood or the same as the subject of the main clause.

• antes de comprar = before buying / before I buy

This is a very common structure:

• antes de sair = before leaving
• antes de dormir = before sleeping
• antes de estudar = before studying

In your sentence, the subject of comprar is understood to be the same person: I.

If you want to be very explicit, Portuguese can also use a clause with a conjugated verb, but antes de + infinitive is extremely natural here.

Because Portuguese, like English, can leave the object unstated when it is obvious from context.

Here, comprar means something like:

• to shop
• to buy things
• to buy what I need

The list already suggests shopping, so the sentence does not need to say exactly what is being bought.

If you wanted, you could make it more explicit:

• antes de comprar as coisas = before buying the things
• antes de fazer compras = before shopping

But the original sentence sounds completely normal as it is.

Yes. Both are natural, but they are slightly different in feel.

• antes de comprar = before buying
• antes de fazer compras = before shopping

The second one is a bit more explicitly about the shopping activity as a whole. The original sentence with comprar is shorter and very natural.

So both work:

• Eu descobri que posso gastar menos quando faço uma lista antes de comprar.
• Eu descobri que posso gastar menos quando faço uma lista antes de fazer compras.

Because quando here expresses something that regularly happens: when(ever) I make a list.

• quando faço uma lista... = when I make a list / whenever I make a list

If you used se, the meaning would shift more toward if:

• se faço uma lista... = if I make a list...

That sounds more conditional or hypothetical. The original sentence is describing a repeated pattern the speaker has noticed, so quando is the better choice.

Absolutely. In fact, I realized is often one of the most natural translations in this kind of sentence.

Possible translations:

• I discovered that I can spend less when I make a list before buying.
• I found out that I can spend less when I make a list before shopping.
• I realized that I can spend less when I make a list before buying.

The exact English wording depends on tone, but in everyday use descobri often means a personal realization, not just a factual discovery.

The original word order is very natural, but Portuguese does allow some flexibility.

Original:

• Eu descobri que posso gastar menos quando faço uma lista antes de comprar.

You could also hear:

• Descobri que, quando faço uma lista antes de comprar, posso gastar menos.

This version emphasizes the condition first: when I make a list...

However, the original order is simpler and more conversational. For a learner, it is a very good model to follow.

Faço is pronounced roughly like FAH-soo in Brazilian Portuguese, though the final o is often reduced and may sound closer to u in many accents.

The ç is called cedilha. It makes the c sound like s before a, o, or u.

So:

• faço sounds with an s sound
• if it were written faco, the pronunciation rule would be different and incorrect

You will see ç in many common words:

• coração
• moço
• começar

It never appears before e or i, because plain c already has the soft sound there.