Se eu reler o texto com calma, acho que encontro erros de ortografia e de pontuação.

Reler means to reread. It is formed from re- + ler (to read).

So:

• ler = to read
• reler = to read again / reread

In the sentence, Se eu reler o texto... means If I reread the text...

Because after se meaning if, when Portuguese refers to a possible future action, it usually uses the future subjunctive.

So:

• Se eu releio... sounds wrong here
• Se eu reler... is the normal form

This structure is very common:

• Se eu tiver tempo... = If I have time...
• Se ela puder vir... = If she can come...
• Se nós encontrarmos erros... = If we find errors...

In your sentence, Se eu reler o texto... means If I reread the text (at some point / in the future).

Yes, eu can often be omitted in Portuguese, but here keeping it is very natural.

You could say:

• Se reler o texto com calma, acho que encontro erros...

But Se eu reler... is clearer because reler in this form does not clearly show the subject by itself. It looks the same as the infinitive, so eu helps avoid ambiguity.

So in this sentence, eu is helpful and very normal.

Yes, for eu, the future subjunctive of reler happens to look exactly like the infinitive:

• infinitive: reler
• future subjunctive with eu: se eu reler

That can be confusing for learners, but it is normal. Portuguese has several verbs where a conjugated form looks identical to the infinitive in some persons.

Com calma literally means with calm, but in natural English it usually means:

• carefully
• slowly
• without rushing
• taking one’s time

So reler o texto com calma means something like:

• reread the text carefully
• go through the text slowly and attentively

It suggests a more thorough review.

The comma separates the initial if-clause from the main clause.

Structure:

• Se eu reler o texto com calma, = conditional clause
• acho que encontro erros... = main clause

This is standard punctuation in Portuguese, just as in English:

• If I reread the text carefully, I think I find errors...

So the comma is expected and correct.

Acho que encontro is a natural way to express a likely result in a fairly immediate or informal way.

All of these are possible, but they feel a little different:

• acho que encontro = I think I’ll find / I think I can find
• acho que vou encontrar = I think I’m going to find
• acho que encontrarei = I think I will find (more formal/literary)

Brazilian Portuguese often uses the present tense to talk about something expected in the near future or as a likely consequence. So acho que encontro erros sounds natural.

That said, many speakers might also say:

• Se eu reler o texto com calma, acho que vou encontrar erros...

That version is also very common.

Because the sentence is not presenting a hypothetical result in the same way English sometimes does with would.

Portuguese often says:

• Se eu reler..., acho que encontro...

rather than forcing a conditional like would find.

If you used encontraria, the tone would become a bit more explicitly hypothetical or formal:

• Se eu reler o texto com calma, acho que encontraria erros...

That is understandable, but less natural in everyday Brazilian Portuguese than acho que encontro or acho que vou encontrar.

Because achar que in an affirmative statement normally takes the indicative when you are stating what you believe to be true.

So:

• Acho que encontro erros = I think I find / will find errors

The subjunctive is more likely after:

• não acho que...
• expressions of doubt
• denial
• uncertainty in certain structures

For example:

• Não acho que eu encontre erros. = I don’t think I’ll find errors.

But in your original sentence, the speaker is positively expressing an opinion, so encontro is appropriate.

Because Portuguese often repeats the preposition to keep the structure clear and balanced.

So:

• erros de ortografia e de pontuação = errors of spelling and of punctuation

This is very natural and elegant in Portuguese.

You may also sometimes see the second de omitted in some contexts, but here repeating it sounds more complete and standard.

Because they are being used in a general category sense.

• erros de ortografia = spelling errors
• erros de pontuação = punctuation errors

Portuguese often omits the article in this kind of expression, just as English does:

• errors of spelling and punctuation
• not necessarily errors of the spelling and the punctuation

So the sentence is talking about types of errors, not specific spelling or specific punctuation already mentioned before.

Here it means find.

The verb encontrar can mean both:

• to find
• to meet

But context makes the meaning clear.

In encontro erros, it can only mean I find errors, not I meet errors.

Yes, it is natural and correct.

It sounds like something a person might say when reviewing a text. A Brazilian speaker might also choose slightly different versions, such as:

• Se eu reler o texto com calma, acho que encontro alguns erros de ortografia e de pontuação.
• Se eu reler o texto com calma, acho que vou encontrar erros de ortografia e pontuação.

But your original sentence is perfectly understandable and idiomatic.