Com tanto stress, eu cheguei a pensar que o exame era impossível.

Cheguei a pensar is more expressive than just pensei.

It suggests something like:

• I even came to think
• I actually ended up thinking
• I got to the point of thinking

So in this sentence, the speaker is saying that the stress was so strong that they reached the point where they thought the exam was impossible.

This structure often adds a sense of:

• surprise
• extremity
• a limit being reached

So eu pensei que o exame era impossível is simpler: I thought the exam was impossible.

But eu cheguei a pensar que o exame era impossível means something closer to: I even came to think that the exam was impossible.

In Portuguese, chegar a + infinitive is a common structure.

Here, chegar a does not mean literally to arrive to. Instead, it means something like:

• to end up
• to come to
• to get to the point of

So:

• cheguei a pensar = I came to think / I ended up thinking
• chegou a dizer isso = he/she even went so far as to say that
• não cheguei a ver o filme = I didn’t actually get to see the film

It is a very useful pattern and often carries the idea of reaching a certain point.

Because the sentence is reporting what the speaker thought at that time, not stating an objective completed fact.

Here, era impossível uses the imperfect and fits well after pensei que when describing how something seemed in that past moment.

Compare:

• Pensei que o exame era impossível.
  = I thought the exam was impossible.

This means that, in the speaker’s mind at that moment, the exam seemed impossible.

If you said foi impossível, it would sound more like you are describing the exam as a completed fact: it was impossible. That usually does not fit as naturally here.

So era is used because it reflects a state or impression in the past.

Here, com introduces the circumstance or condition in which something happened.

So Com tanto stress means something like:

• With so much stress
• Under so much stress
• Given so much stress

It does not mean possession here. It sets the background for the main clause.

Other similar examples:

• Com tanta pressão, ele desistiu.
  = With so much pressure, he gave up.
• Com tanto trabalho, não tive tempo.
  = With so much work, I didn’t have time.

In English, we might often prefer with, under, or because of, depending on context.

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

• muito stress = a lot of stress
• tanto stress = so much stress

Tanto is stronger and often sounds more emotional or emphatic. It helps explain why the speaker reached that point of thinking the exam was impossible.

So:

• Com muito stress = neutral
• Com tanto stress = more intense, more expressive

That makes tanto a very natural choice here.

No, eu is not grammatically necessary.

Portuguese often drops subject pronouns when the verb already makes the subject clear.

So both are correct:

• Com tanto stress, eu cheguei a pensar que o exame era impossível.
• Com tanto stress, cheguei a pensar que o exame era impossível.

The version with eu can add:

• emphasis
• contrast
• personal involvement

So using eu is perfectly natural, but leaving it out is also very common.

Yes. Stress is widely used in European Portuguese.

It is a common borrowed word and sounds natural in everyday speech. In this sentence, it means psychological pressure or tension, just like in English.

You may also see other words depending on context, such as:

• pressão = pressure
• tensão = tension
• nervosismo = nervousness

But stress is extremely common and natural here.

Because stress is being used as an uncountable noun, like stress in English.

So:

• com tanto stress = with so much stress

This is similar to:

• com tanto trabalho = with so much work
• com tanta pressão = with so much pressure

You can use an article in other contexts, for example:

• O stress do trabalho está a afetá-lo.
  = Work stress is affecting him.

But in this sentence, no article is needed.

Exame is a masculine noun in Portuguese, so it takes o in the singular:

• o exame = the exam
• os exames = the exams

This is just the normal definite article agreeing with the noun.

After pensar que, Portuguese normally uses the indicative when you are expressing a belief or thought as content.

So:

• Pensei que o exame era impossível.

This is normal because the clause after que is the thing the speaker thought.

The subjunctive is more likely in negative or doubtful structures, for example:

• Não penso que seja impossível.
  = I don’t think it is impossible.

So in your sentence, era in the indicative is exactly what we expect.

Not really. That would sound unnatural in English.

Although chegar literally means to arrive, the expression chegar a + infinitive is idiomatic. You should usually translate it with something more natural in English, such as:

• to come to
• to end up
• to even
• to get to the point of

So the sentence is better understood as:

• With so much stress, I even came to think that the exam was impossible.
• With so much stress, I ended up thinking the exam was impossible.

Yes. Portuguese word order is fairly flexible.

For example, these are all possible:

• Com tanto stress, eu cheguei a pensar que o exame era impossível.
• Eu cheguei a pensar, com tanto stress, que o exame era impossível.
• Cheguei a pensar, com tanto stress, que o exame era impossível.

The original version sounds very natural because Com tanto stress is placed first to set the scene.

So the sentence begins by giving the reason or background, and then tells what happened mentally.

It could, but the meaning shifts a bit.

• era impossível suggests that the speaker saw the exam as impossible in a more general sense
• estava impossível suggests that it seemed impossible at that moment or under those conditions

So:

• Pensei que o exame era impossível = I thought the exam was impossible
• Pensei que o exame estava impossível = I thought the exam was impossibly hard / seemed impossible at that time

In many contexts, both could work, but era impossível is the more straightforward choice in this sentence.

It works in both European and Brazilian Portuguese.

Nothing in the sentence is strongly limited to Portugal. A Brazilian speaker would also understand it easily, and many would say it naturally as well.

That said, pronunciation and some stylistic preferences may vary, but grammatically and semantically the sentence is fine in both varieties.