Meine Freundin wundert sich über denselben Fehler im Dokument wie gestern.

Because the verb here is sich wundern über + accusative, which means to be surprised about / to wonder at something.

So:

• Meine Freundin wundert sich = My girlfriend is surprised
• über denselben Fehler = about the same mistake

In German, many verbs are used with a reflexive pronoun even when English does not use one. Here, sich is not translated as herself in normal English; it is just part of the verb pattern.

For different subjects, the reflexive pronoun changes:

• ich wundere mich
• du wunderst dich
• er/sie wundert sich
• wir wundern uns

So in this sentence, sich matches meine Freundin.

Because sich wundern commonly takes the preposition über when you say what someone is surprised about.

Pattern:

• sich wundern über etwas

Examples:

• Ich wundere mich über das Wetter.
  = I’m surprised about the weather.
• Sie wundert sich über den Preis.
  = She’s surprised about the price.

This is something you mostly have to learn with the verb. English speakers often want to translate word-for-word, but in German the preposition used with a verb is often fixed.

There are two things happening here:

1. Fehler is masculine: der Fehler
2. After über, we need the accusative here

So der Fehler becomes den Fehler in the accusative.

Then derselbe changes too, because it behaves like a determiner with endings that depend on case, gender, and number.

For masculine accusative:

• derselbe Fehler = the same mistake
• über denselben Fehler = about the same mistake

That is why you get denselben.

A quick mini-table for derselbe:

• nominative masculine: derselbe
• accusative masculine: denselben
• dative masculine: demselben

This is a very common learner question.

Traditionally:

• derselbe Fehler = the very same mistake
  (literally the identical one)
• der gleiche Fehler = the same kind of mistake
  (a similar one, not necessarily the exact identical instance)

In real everyday German, many speakers use derselbe and der gleiche less strictly than grammar books suggest, and der gleiche Fehler is also very common.

Still, if you want the textbook distinction:

• derselbe = one and the same
• der gleiche = an equivalent/similar one

So this sentence suggests she is surprised about the exact same error as before.

Im is a contraction of in dem.

• in dem → im

Here, in describes a location, not movement, so German uses the dative:

• im Dokument = in the document

Compare:

• im Dokument = in the document, inside it, located there
• in das Dokument = into the document, movement into it

Since the error is located in the document, not moving into it, im Dokument is correct.

Because in is a two-way preposition (Wechselpräposition). That means it can take either:

• dative for location
• accusative for direction/movement

Here the meaning is static location:

• the error is in the document

So we use the dative:

• in dem Dokument
• contracted: im Dokument

If the sentence involved movement, accusative would be possible:

• Ich lege das Blatt in das Dokument.
  = I put the sheet into the document.

But in your sentence, no movement is involved.

Wie gestern means as yesterday or like yesterday.

It compares the current situation with yesterday’s situation. In natural English, the idea is something like:

• My girlfriend is surprised about the same mistake in the document as yesterday.
• or more naturally: My girlfriend is surprised about the same mistake in the document, just like yesterday.

It is a shortened comparison. German often leaves out words that are understood from context.

You can think of it as something like:

• ... über denselben Fehler im Dokument wie gestern
  = ... about the same error in the document as the one from yesterday / as yesterday

The phrase attaches to the idea of sameness/comparison.

Yes, a little.

A learner might wonder whether it means:

1. she is surprised today, just as she was yesterday, or
2. it is the same mistake as yesterday

In real context, the sentence most naturally suggests a comparison with yesterday involving the same error and probably also the repeated surprise. German often allows this kind of compact phrasing when the meaning is clear from context.

If you wanted to make it clearer, you could rewrite it in different ways, for example:

• Meine Freundin wundert sich wie gestern über denselben Fehler im Dokument.
  This puts more focus on just like yesterday, she is surprised...
• Meine Freundin wundert sich über denselben Fehler im Dokument wie gestern schon.
  This strengthens the connection to yesterday’s earlier surprise.

So yes, there is some possible ambiguity, but in normal use the intended meaning is usually easy to understand.

This is normal German main-clause word order:

• the finite verb comes in second position
• the subject often comes first, but not always

So the structure is:

• Meine Freundin = subject
• wundert = finite verb in second position
• sich = reflexive pronoun
• then the rest of the sentence

German does not keep all parts of the verb together the way English sometimes does. The reflexive pronoun usually comes shortly after the finite verb.

Basic structure here:

• Meine Freundin | wundert | sich | über denselben Fehler im Dokument | wie gestern

You could also move other elements to the front for emphasis:

• Über denselben Fehler im Dokument wundert sich meine Freundin wie gestern.

That sounds more marked, but it is grammatically possible.

In a normal main clause, the finite verb is in second position, and the reflexive pronoun usually comes after it.

So:

• Sie wundert sich.

Not:

• Sie sich wundert. in a main clause

However, in a subordinate clause, the verb moves to the end:

• ..., weil sie sich wundert.

So English speakers should remember:

• main clause: verb second
  Sie wundert sich.
• subordinate clause: verb later/often final
  ..., weil sie sich über den Fehler wundert.

Yes. Freundin can mean either:

• girlfriend
• female friend

German does not automatically distinguish those two meanings in the word itself.

Context usually makes it clear. If someone wants to avoid ambiguity, they might say:

• eine Freundin von mir = a female friend of mine
• meine feste Freundin = my steady girlfriend / romantic partner

So depending on context, Meine Freundin could be understood in either way.

Because the sentence is talking about one specific error:

• denselben Fehler = the same error / mistake

If it were plural, it would be:

• dieselben Fehler = the same errors

And with the full phrase:

• über dieselben Fehler im Dokument

The singular suggests there is one particular repeated problem being referred to.

In standard modern German, the usual spelling is:

• denselben Fehler

written as one word.

That is because derselbe / dieselbe / dasselbe and its inflected forms are generally treated as single-word forms.

So in your sentence, standard spelling is:

• über denselben Fehler

Not exactly.

• sich wundern über etwas = to be surprised about something
• sich fragen = to ask oneself / to wonder in the sense of thinking about a question

Examples:

• Ich wundere mich über den Lärm.
  = I’m surprised about the noise.
• Ich frage mich, warum es so laut ist.
  = I wonder why it is so loud.

So wundern is about surprise, while sich fragen is about thinking/asking oneself.

In your sentence, wundert sich über clearly means is surprised about.