Foi uma surpresa ver a Ana no teatro, porque ninguém esperava por ela.

Foi is the preterite form of ser, and here it presents the surprise as a completed reaction to a specific event.

• Foi uma surpresa ver a Ana no teatro = It was a surprise to see Ana at the theater.
• It refers to one particular moment: seeing her there.

If you said Era uma surpresa, it would usually sound less natural here, because era often gives background description or an ongoing state in the past, not a single completed reaction.

So in this sentence, foi is the normal choice.

Portuguese usually uses articles more often than English does. So uma surpresa is the natural way to say a surprise or something surprising in this context.

• Foi surpresa can exist in some contexts, but it is less neutral here.
• Foi uma surpresa sounds complete and natural.

So even if English sometimes handles this a little differently, in Portuguese the article is expected.

The a before Ana is the definite article.

In Brazilian Portuguese, it is very common to use articles before people's names:

• a Ana
• o João
• a Maria

So ver a Ana literally looks like to see the Ana, but in natural English we just say to see Ana.

This article is very common in Brazil, although usage varies by region and style.

No, it is common, but not always required.

Both of these are possible:

• Ver a Ana
• Ver Ana

In many parts of Brazil, using the article sounds more natural in everyday speech. In more formal writing, or depending on the region, speakers may omit it more often.

So for a learner:

• a Ana = very natural in spoken Brazilian Portuguese
• Ana = also possible

No is a contraction of:

• em + o = no

So:

• no teatro = in the theater / at the theater

This kind of contraction is extremely common in Portuguese:

• em + a = na
• em + os = nos
• em + as = nas

So no teatro is just the normal contracted form.

Because ninguém is grammatically singular.

Even though it refers to no person / nobody, the verb stays in the singular:

• ninguém esperava
• ninguém sabia
• ninguém viu

This is similar to English nobody was, not nobody were.

Here esperava is the imperfect past.

The imperfect is often used for:

• ongoing past situations
• background information
• states or expectations that existed at that time

In this sentence, ninguém esperava por ela means that, at that moment/situation, nobody was expecting her.

If you used esperou, it would sound more like a completed action:

• Ninguém esperou por ela = Nobody waited for her or Nobody expected her in a more bounded/completed sense, depending on context.

But here the idea is about a general expectation in the background, so esperava fits better.

With esperar, Portuguese can use two patterns:

• esperar alguém
• esperar por alguém

In many varieties of Brazilian Portuguese, esperar por is very common, especially in everyday speech.

Depending on context, esperar can mean:

• to wait for
• to expect

In this sentence, esperava por ela means was expecting her.

So the por is part of a very common verb pattern here.

In real spoken Brazilian Portuguese, you may hear forms like esperava ela, but for learners the safer and more standard options are:

• ninguém esperava ela — possible in some speech, but less standard
• ninguém a esperava — more formal/written style
• ninguém esperava por ela — very natural and common in Brazil

So in this sentence, esperava por ela is an excellent, natural choice.

Because once Ana has already been mentioned, Portuguese can use the pronoun ela to avoid repetition.

• ...ver a Ana no teatro, porque ninguém esperava por ela.

This works just like English:

• ...to see Ana at the theater, because nobody was expecting her.

It makes the sentence flow more naturally.

The comma separates the main idea from the explanation:

• Foi uma surpresa ver a Ana no teatro
• porque ninguém esperava por ela

The second part explains why it was a surprise.

In Portuguese, a comma before porque is common when the second clause adds an explanation to the first. You will often see this kind of punctuation in sentences like this.

Here porque means because.

Portuguese has several similar-looking forms:

• porque = because
• por que = often why / for what reason
• porquê = the reason
• por quê = why? at the end of a question

In your sentence, it is clearly porque = because.

Literally, no teatro is in the theater, but in natural English the best translation is often at the theater, depending on context.

Portuguese em / no / na often covers meanings that English expresses with either in, at, or sometimes on, depending on the situation.

So both ideas are possible, but the natural English translation is often at the theater.

The sentence breaks down like this:

• Foi uma surpresa = It was a surprise
• ver a Ana no teatro = to see Ana at the theater
• porque ninguém esperava por ela = because nobody was expecting her

So the overall structure is:

[main statement] + [infinitive phrase] + [reason clause]

That is:

Foi uma surpresa

• ver a Ana no teatro
• porque ninguém esperava por ela

This is a very common and useful sentence pattern in Portuguese.