Sem a garantia, eu teria que pagar para consertar esse defeito; por isso li o manual com atenção.

Both are possible, but they are not exactly the same.

• sem a garantia = without the warranty / without that warranty
• sem garantia = without warranty / with no guarantee

In your sentence, a garantia most naturally refers to a specific warranty already understood in context, such as the product warranty. So sem a garantia sounds like without the warranty coverage.

Also, garantia is feminine, so the article is a.

Yes, eu could be omitted:

• Sem a garantia, teria que pagar...

Portuguese often drops subject pronouns when the verb form already makes the subject clear. Here, teria clearly points to eu.

However, including eu can:

• add emphasis,
• make the sentence clearer,
• sound a bit more explicit or contrastive.

So both are correct:

• Sem a garantia, eu teria que pagar...
• Sem a garantia, teria que pagar...

teria que means would have to.

It is built from:

• teria = conditional of ter
• que = introducing the obligation

So:

• eu teria que pagar = I would have to pay

This is a very common way to express obligation in a hypothetical situation.

You may also see:

• eu teria de pagar

That means basically the same thing. In everyday Brazilian Portuguese, ter que is extremely common.

Because the sentence is talking about a hypothetical situation, not a real past obligation.

• teria que pagar = would have to pay
• tinha que pagar = had to pay / used to have to pay

Here, Sem a garantia... creates a hypothetical condition: if I didn’t have the warranty. So the conditional teria is the natural choice.

Compare:

• Sem a garantia, eu teria que pagar.
  = Without the warranty, I would have to pay.

• Sem a garantia, eu tinha que pagar.
  = Without the warranty, I had to pay.
  This sounds like a real past situation, not a hypothetical one.

In Portuguese, pagar para + infinitive is a normal way to say pay to do something.

So:

• pagar para consertar esse defeito = pay to fix that defect

Using por here would usually not be the natural choice.

A useful pattern is:

• pagar para fazer algo = pay to do something
• pagar por algo = pay for something

Examples:

• Paguei para arrumar o carro. = I paid to fix the car.
• Paguei pelo conserto do carro. = I paid for the repair of the car.

So in your sentence, pagar para consertar focuses on the action; pagar pelo conserto would focus more on the repair as a thing/service.

Yes, consertar means to fix, repair.

And yes, learners often confuse it with:

• consertar = to repair
• concertar = to arrange, harmonize, or in some contexts related to a concert / agreement

So here the correct spelling is definitely consertar, because it refers to repairing a problem.

Examples:

• consertar o carro = fix the car
• consertar um erro = fix an error

In Brazilian Portuguese, esse is very common in everyday speech and writing.

Traditionally, the distinction is:

• este = this, near the speaker
• esse = that, near the listener or something already mentioned
• aquele = that over there, farther away

But in modern Brazilian Portuguese, especially in everyday use, esse often covers situations where English would use this or that.

So esse defeito is completely natural. It can mean:

• this defect
• that defect

depending on context.

A learner should know the formal distinction, but also know that real Brazilian usage is often less strict.

Defeito often means:

• defect
• fault
• problem
• flaw

For a product or machine, it often means some kind of malfunction or imperfection.

So depending on context, English translations could be:

• this defect
• this fault
• this problem

In a sentence about a warranty, defeito strongly suggests a product flaw or technical problem.

por isso means:

• for that reason
• because of that
• so
• therefore

In your sentence:

• por isso li o manual com atenção = for that reason, I read the manual carefully

This is different from porque, which usually means because and introduces a reason directly.

Compare:

• Li o manual com atenção porque queria entender a garantia.
  = I read the manual carefully because I wanted to understand the warranty.

• Sem a garantia, eu teria que pagar...; por isso li o manual com atenção.
  = Without the warranty, I would have to pay...; so I read the manual carefully.

So:

• porque introduces the reason
• por isso points back to a previously stated reason and means therefore / so

li is the first-person singular preterite of ler (to read).

So:

• eu li = I read / I did read

This is a completed past action.

Conjugation of ler in the preterite:

• eu li
• você/ele/ela leu
• nós lemos
• vocês/eles/elas leram

Learners often notice that li is short and a bit irregular-looking. That is just how this verb works in the past.

Yes, approximately.

li is pronounced roughly like lee in English, but with a pure Portuguese vowel sound.

So:

• li ≈ lee

It is not pronounced like English lie.

com atenção literally means with attention, but in natural English it usually means:

• carefully
• attentively

So:

• li o manual com atenção = I read the manual carefully

This is a common Portuguese structure:

• ouvir com atenção = listen carefully
• ler com atenção = read carefully
• prestar atenção = pay attention

The semicolon links two closely related ideas:

1. Sem a garantia, eu teria que pagar para consertar esse defeito
2. por isso li o manual com atenção

The second idea is a consequence of the first. A semicolon works well here because the connection is strong.

You could also write:

• Sem a garantia, eu teria que pagar para consertar esse defeito. Por isso, li o manual com atenção.
• Sem a garantia, eu teria que pagar para consertar esse defeito, por isso li o manual com atenção.

The semicolon makes the relationship especially clear and neat in writing.

Yes, li o manual com atenção sounds very natural.

Portuguese often prefers a prepositional phrase like:

• com atenção
• com cuidado

instead of a heavier adverb.

So these are all possible:

• Li o manual com atenção.
• Li o manual com cuidado.
• Li o manual cuidadosamente.

The first one is very common and natural in Brazilian Portuguese.

Yes. garantia can mean both guarantee and warranty, depending on context.

In a sentence about paying to repair a product defect, the most natural English translation is usually warranty.

So here:

• Sem a garantia most likely means Without the warranty

If the context were broader or more abstract, garantia could also be guarantee.

Yes, a few things fit very naturally with Brazilian Portuguese usage:

• esse being used in a broad everyday way
• ter que being very common for have to
• com atenção sounding natural and idiomatic

Nothing in the sentence is strange or overly formal. It sounds like normal, clear Brazilian Portuguese.