Der Fehler kompliziert die Operation.

Because in this sentence kompliziert is the 3rd person singular present form of the verb komplizieren = to complicate.

• komplizieren = infinitive
• er/sie/es kompliziert = he/she/it complicates

So:

• Der Fehler kompliziert die Operation. = The error complicates the operation.

It looks exactly like the adjective kompliziert = complicated, which can be confusing. Compare:

• Die Operation ist kompliziert. = The operation is complicated.
  Here kompliziert is an adjective.
• Der Fehler kompliziert die Operation. = The error complicates the operation.
  Here kompliziert is a verb.

The position helps: in a normal German main clause, the finite verb is usually in second position.

These are the nouns’ grammatical genders:

• Fehler is masculine → der Fehler
• Operation is feminine → die Operation

In German, every noun has a grammatical gender, and you usually have to learn it together with the noun:

• der Fehler
• die Operation

The gender does not necessarily match natural gender or make logical sense to English speakers; it is just part of the word.

In this sentence:

• Der Fehler = the subject
• die Operation = the direct object

There are two main clues:

1. Word order
German main clauses normally put the subject before the verb when nothing special is being emphasized:

• Der Fehler | kompliziert | die Operation

So the most natural reading is: The error complicates the operation.

2. Case marking
Der Fehler shows nominative masculine: der
That strongly suggests it is the subject.

Die Operation is feminine, and die can be either nominative or accusative singular. So by itself, die Operation does not tell you as clearly whether it is subject or object. But with der Fehler in nominative and the normal word order, the meaning is clear.

In German, very often the article shows the case more clearly than the noun itself.

Here:

• der Fehler → nominative masculine
• die Operation → accusative feminine

The noun Fehler stays the same in nominative and accusative singular. That is normal for many masculine and neuter nouns.

So learners often need to look at the article, not only the noun:

• der Fehler = subject form here
• den Fehler = accusative form

For example:

• Der Fehler ist schlimm. = The error is bad.
• Ich sehe den Fehler. = I see the error.

Because German main clauses follow the verb-second rule for the finite verb.

In a normal statement, the conjugated verb comes in second position:

• Der Fehler | kompliziert | die Operation

This is one of the most important German word-order rules.

Even if you move another element to the front, the finite verb still stays second:

• Die Operation kompliziert der Fehler.
  This is possible, but more marked and less neutral.

The basic version for learners is:

• Subject + finite verb + rest

Not always. Operation can mean several things depending on context, such as:

• a surgical operation
• a military operation
• a general operation/procedure

So the exact meaning depends on the situation. If your translation already gives the intended meaning, that is the meaning to use here.

English speakers should be careful not to assume it always means only one thing.

Yes, it exists and is correct, but it can sound somewhat formal depending on context.

German also often uses other ways to express the same idea, such as:

• etwas schwierig machen = to make something difficult
• etwas komplizierter machen = to make something more complicated
• etwas erschweren = to make something harder / hinder

So Der Fehler kompliziert die Operation is grammatically fine, but in everyday speech a speaker might sometimes choose a different phrasing depending on style and context.

Because all nouns are capitalized in German.

So:

• der Fehler
• die Operation

This is a basic spelling rule in German and one of the easiest ways to spot nouns in a sentence.

The infinitive is komplizieren.

Here is the present tense:

• ich kompliziere
• du komplizierst
• er/sie/es kompliziert
• wir komplizieren
• ihr kompliziert
• sie/Sie komplizieren

In your sentence, the subject der Fehler is third person singular, so the correct form is:

• Der Fehler kompliziert die Operation.

Yes, possibly. Fehler can often mean error, mistake, or fault, depending on context.

Common possibilities are:

• error
• mistake
• fault

Which English word fits best depends on the situation:

• a human action → often mistake
• a technical/system issue → often error or fault

So if the meaning is already given to the learner, use that meaning for this sentence, but it is good to know that Fehler is flexible in translation.