Eu faltei à reunião porque estava com febre.

Because faltar a means to miss / to be absent from something.

• Eu faltei à reunião = I missed the meeting / I was absent from the meeting
• Eu fui à reunião = I went to the meeting

So the verb changes the meaning completely.

À is a contraction of a + a:

• the preposition a
• plus the feminine singular definite article a

The verb faltar normally uses the preposition a:

• faltar a uma reunião
• faltar à reunião = miss the meeting

Because reunião is feminine and has the article a, the two combine into à.

The accent in à marks a contraction:

• a = just a preposition, or sometimes an article depending on context
• à = a + a

So in faltei à reunião, the accent shows that the preposition and article have merged.

This is called crase in Portuguese.

Yes, it can be omitted.

Portuguese often leaves out subject pronouns when the verb already makes the subject clear:

• Faltei à reunião porque estava com febre.
• Eu faltei à reunião porque estava com febre.

Both are natural. Including eu can add emphasis, contrast, or clarity.

Faltei is the preterite of faltar.

It refers to a completed action in the past:

• Eu faltei = I missed / I was absent

Here it describes the event of missing the meeting as a finished past action.

Estava is the imperfect of estar.

It is used because the fever is presented as a background or ongoing condition at that time:

• porque estava com febre = because I had a fever / because I was running a fever

So the sentence combines:

• faltei (preterite) for the main completed event
• estava (imperfect) for the ongoing state or circumstance that explains it

This combination is very common in Portuguese.

Estava still refers to eu.

Portuguese often omits the subject pronoun when it is understood from context. Since the sentence starts with Eu faltei, it is natural to understand that the same person also estava com febre.

So:

• Eu faltei à reunião porque eu estava com febre.
• Eu faltei à reunião porque estava com febre.

Both mean the same thing, but the second is more natural and less repetitive.

In Brazilian Portuguese, estar com febre is the most natural way to say to have a fever or to be running a fever.

Literally, it is to be with fever, but that is just how Portuguese expresses temporary physical conditions:

• estar com febre = to have a fever
• estar com dor de cabeça = to have a headache
• estar com sono = to be sleepy

You may hear tinha febre in some contexts, but estava com febre is the more typical everyday phrasing here.

Using à reunião means the meeting, usually a specific meeting already known from context.

• Faltei à reunião = I missed the meeting
• Faltei a uma reunião = I missed a meeting

So the definite article makes it sound like a particular meeting, not just any meeting.

Usually, porque is the most natural choice here.

• porque estava com febre = because I had a fever

You could sometimes use pois, but it often sounds more formal, literary, or stylistically different:

• Faltei à reunião, pois estava com febre.

That is grammatical, but porque is the normal everyday word for because in this sentence.

Yes, it is completely natural.

A Brazilian speaker would easily say:

• Eu faltei à reunião porque estava com febre.

Even more naturally in many contexts, they might drop eu:

• Faltei à reunião porque estava com febre.

Both are standard and idiomatic.

It would not change.

Portuguese verbs usually do not change according to the speaker’s gender in this kind of sentence:

• a man: Eu faltei à reunião porque estava com febre.
• a woman: Eu faltei à reunião porque estava com febre.

The sentence stays exactly the same.