Eu consegui terminar o relatório antes da reunião.

Yes. In Brazilian Portuguese, subject pronouns are often omitted because the verb form already shows who the subject is.

So:

• Eu consegui terminar o relatório antes da reunião.
• Consegui terminar o relatório antes da reunião.

Both are natural.

Keeping eu can add:

• emphasis: I managed to finish it
• contrast: I finished it, but someone else didn’t
• clarity in some contexts

Consegui is the preterite (simple past) of conseguir, in the first person singular.

• conseguir = to manage, to succeed, to be able to
• consegui = I managed / I was able

In this sentence, it refers to a completed past action: the speaker successfully finished the report.

Because after conseguir, Portuguese commonly uses another verb in the infinitive to say what someone managed to do.

So the structure is:

• conseguir + infinitive
• consegui terminar = I managed to finish

Other examples:

• Consegui chegar cedo. = I managed to arrive early.
• Conseguimos resolver o problema. = We managed to solve the problem.

Both can be translated naturally into English, but they are not exactly the same.

• Eu terminei o relatório. = I finished the report.
• Eu consegui terminar o relatório. = I managed to finish the report.

Consegui terminar suggests that finishing it may have required effort, time, or overcoming some difficulty. It highlights the success of achieving it, not just the fact that it happened.

They are related, but not identical.

• consegui terminar strongly suggests successful completion
• pude terminar can mean I was able to finish or I had the opportunity/possibility to finish

In many contexts, consegui terminar is the safer choice if you want to make it clear that the report really was finished.

Compare:

• Consegui terminar o relatório. = I managed to finish the report.
• Pude terminar o relatório. = I was able to finish the report / I got the chance to finish the report.

Portuguese uses definite articles more often than English.

• o relatório = the report

In this sentence, it refers to a specific report, so the article is natural and expected.

English sometimes omits articles where Portuguese would not, but here both languages use one:

• the report
• o relatório

Because de + a contracts in Portuguese:

• de + a = da

And after antes, Portuguese normally uses de. So:

• antes de a reunião → antes da reunião

This contraction is standard and very common.

Other examples:

• do = de + o
• dos = de + os
• das = de + as

Because antes is normally followed by de in Portuguese.

So the basic pattern is:

• antes de + noun
• antes de + infinitive

Examples:

• antes da reunião = before the meeting
  (de + a reunião = da reunião)
• antes de sair = before leaving

So in your sentence, antes da reunião literally comes from antes de a reunião, but it must contract to da reunião.

Usually not in this context.

Antes da reunião refers to a specific meeting: before the meeting.

Antes de reunião sounds incomplete or unnatural in most normal situations, unless you are speaking in a very general or institutional way. For a normal sentence about one particular meeting, antes da reunião is the natural choice.

Yes, somewhat.

These are all possible depending on context:

• Eu consegui terminar o relatório antes da reunião.
• Consegui terminar o relatório antes da reunião.
• Antes da reunião, eu consegui terminar o relatório.

The original order is very natural and neutral. Moving antes da reunião to the front gives more focus to the time phrase.

Because reunião is a feminine noun in Portuguese, so it takes a in the singular:

• a reunião = the meeting

That is why:

• antes da reunião = before the meeting
  (de + a reunião = da reunião)

Grammatical gender in Portuguese does not always match any logical rule, so nouns usually have to be learned with their article:

• o relatório
• a reunião

They mainly show stress and, in some cases, vowel quality.

• relatório: the accent shows the stressed syllable: re-la-TÓ-rio
• reunião: the ã shows a nasal sound, and -ão is a very common Portuguese ending

So the accents are not optional spelling marks; they are part of the correct written form of the words.

Very often, yes. That is one of the most useful translations.

• consegui terminar = I managed to finish
• consegui fazer = I managed to do
• conseguiu chegar = he/she managed to arrive

But depending on context, it can also be translated as:

• was able to
• succeeded in

The key idea is successful ability or achievement.