El encuentro nos sorprendió.

Because encuentro is being used as a noun here: the encounter / the meeting.

In Spanish, nouns often appear with an article where English might sometimes leave it out. Here, el encuentro refers to a specific encounter or meeting, not just any encounter in general.

Also, encuentro is a masculine singular noun, so it takes el.

Yes. Here it is definitely a noun.

Why we know:

• It comes after el, which is an article.
• The verb of the sentence is sorprendió.
• So the structure is:
  • El encuentro = the encounter / meeting
  • nos = us
  • sorprendió = surprised

So this is not encuentro as the yo form of encontrar.

Here, nos means us.

It is the object pronoun referring to the people who were surprised.

So:

• El encuentro = the thing that caused the surprise
• nos = the people affected
• sorprendió = surprised

In other words, the sentence structure is basically:

The encounter surprised us.

It is a direct object here.

That can feel a little confusing for English speakers, because in English we sometimes think of emotions as happening to someone. But with sorprender, Spanish treats the person as the direct object:

• La noticia me sorprendió. = The news surprised me.
• El encuentro nos sorprendió. = The encounter surprised us.

So nos is not to us grammatically — it is us.

Because unstressed object pronouns in Spanish normally go before a conjugated verb.

So Spanish says:

• nos sorprendió

not:

• sorprendió nos

This is a standard word order rule with pronouns like:

• me
• te
• lo / la
• nos
• os
• los / las

With a normal finite verb, these pronouns usually go right before it.

Sorprendió is the preterite tense, third person singular, of sorprender.

That means it refers to a completed event in the past:

• sorprendió = surprised

The subject is singular (el encuentro), so the verb is also singular.

Compare:

• El encuentro nos sorprendió. = The encounter surprised us.
• Los encuentros nos sorprendieron. = The encounters surprised us.

The accent mark shows the correct stress: sor-pren-di-Ó.

It is important because without the accent, the written form would not follow the intended pronunciation. In the preterite, many third-person singular forms ending in -ió carry a written accent:

• comió
• vivió
• sorprendió

So the accent helps mark both pronunciation and the correct verb form.

The subject is El encuentro.

A very common learner mistake is to see nos near the verb and think it might be the subject, but it is not. Nos is the object pronoun.

You can test it by looking at verb agreement:

• El encuentro is singular
• the verb is sorprendió, also singular

So:

• subject: el encuentro
• object: nos

Yes, absolutely.

Both of these are correct:

• El encuentro nos sorprendió
• Nos sorprendió el encuentro

The basic meaning stays the same, but the emphasis shifts a little.

• El encuentro nos sorprendió starts with the subject, so it feels more neutral.
• Nos sorprendió el encuentro puts nos sorprendió first, which can sound slightly more natural in some contexts or give a bit more focus to the reaction.

Spanish word order is more flexible than English word order.

You could, but the meaning would change.

• nos sorprendió = it surprised us
  
  • a completed past event
• nos sorprendía = it was surprising us / it used to surprise us / it would surprise us
  
  • more ongoing, repeated, or backgrounded in the past

So in your sentence, sorprendió presents the encounter as a single completed event.

It can mean several related things depending on context, for example:

• encounter
• meeting
• match or game in sports contexts

So el encuentro could refer to:

• a meeting between people
• an unexpected encounter
• even a football match, depending on the situation

The grammar of the sentence stays the same either way.

Because nos is a pronoun, not a full noun phrase like a Juan or a mis amigos.

The personal a is used before specific human direct objects when they are expressed as nouns:

• Vi a Juan.

But with object pronouns, you just use the pronoun itself:

• Nos sorprendió.
• Me vio.

So nos does not need a here.

It is similar in meaning only in the sense that English and Spanish organize the idea differently from what some learners expect.

With sorprender, the thing causing the feeling is the subject:

• El encuentro nos sorprendió.
  = The encounter surprised us.

That is actually quite natural in Spanish and close to English grammar.

With gustar, the structure is different:

• Nos gustó el encuentro.
  literally: The encounter pleased us

So sorprender is not exactly the same as gustar, but both can make learners stop and think about who is doing what in the sentence.