Der Techniker prüft den Verdichter.

Because der Techniker is the subject of the sentence, and den Verdichter is the direct object.

• der Techniker = nominative case → the person doing the action
• den Verdichter = accusative case → the thing receiving the action

In this sentence:

• Der Techniker = the technician does the checking
• prüft
• den Verdichter = the compressor / condenser / compactor (depending on context) is being checked

For a masculine noun, the definite article changes like this:

• der in the nominative
• den in the accusative

That is why the two articles are different.

You know mainly because of the case marking.

In English, we usually depend heavily on word order:

• The technician checks the compressor ≠ The compressor checks the technician

In German, word order matters too, but the articles often make the roles clearer.

Here:

• der Techniker = nominative = subject
• den Verdichter = accusative = object

So even if the sentence were rearranged, the articles would still show who is doing what:

• Den Verdichter prüft der Techniker.

This still means The technician checks the compressor, not the other way around.

prüft is the 3rd person singular present tense form of the verb prüfen, meaning to check, to inspect, or to test.

Conjugation of prüfen in the present tense:

• ich prüfe
• du prüfst
• er/sie/es prüft
• wir prüfen
• ihr prüft
• sie/Sie prüfen

Because der Techniker is he/it grammatically treated as 3rd person singular, the verb is prüft.

Because in this sentence, the article shows the case clearly, so the noun itself often stays the same.

You have:

• der Techniker in the nominative
• If it were accusative, it would still usually be den Techniker

So the noun Techniker does not change much here; the article does most of the work.

This is very common in German. Compare:

• der Mann → den Mann
• der Techniker → den Techniker

With many masculine nouns, the article changes more obviously than the noun itself.

Because all nouns are capitalized in German.

So:

• Techniker is a noun
• Verdichter is a noun

That is why both start with capital letters.

This is one of the most noticeable spelling rules in German. It applies not just to names of people, but to all nouns:

• der Tisch
• die Maschine
• das Auto
• der Techniker
• der Verdichter

In this sentence, yes, Techniker is masculine, as shown by der. But the ending -er does not automatically guarantee masculine gender in every German noun.

Still, for job titles referring to a male person, Techniker is masculine:

• der Techniker = the male technician
• die Technikerin = the female technician

If you are speaking about a specific man, der Techniker is correct.

If you mean the profession in a general or mixed sense, usage can vary depending on style and context.

Yes, German word order is more flexible than English, but there are rules.

The basic sentence is:

• Der Techniker prüft den Verdichter.

You can also say:

• Den Verdichter prüft der Techniker.

This puts more emphasis on den Verdichter.

However, in a normal main clause, the conjugated verb usually stays in second position. That is the key rule.

So these are possible:

• Der Techniker prüft den Verdichter.
• Heute prüft der Techniker den Verdichter.
• Den Verdichter prüft der Techniker.

But the verb prüft still stays in the second slot of the clause.

Because in normal German, nouns usually need an article or some other determiner.

So German strongly prefers:

• Der Techniker prüft den Verdichter.

Without articles, the sentence sounds incomplete or unnatural in most ordinary contexts.

German uses articles more often than English in many cases. Even where English sometimes omits them, German often keeps them.

Using the articles also helps show:

• gender
• case
• sentence role

That is especially useful here because der and den show subject and object.

The hardest part for many English speakers is ü.

A practical approximation:

• Say ee as in see
• While keeping that tongue position, round your lips as if saying oo

That gives you something close to German ü.

So prüft is roughly:

• prüft → one syllable
• starts with pr
• then ü
• ends with ft

The f and t are both clearly pronounced.

It is not like English proofed, even though that may look tempting to an English speaker.

The sentence has a very standard German structure:

• Der Techniker = subject noun phrase in the nominative
• prüft = finite verb, present tense, 3rd person singular
• den Verdichter = direct object noun phrase in the accusative

So the pattern is:

Subject + Verb + Direct Object

This is one of the most basic and useful sentence patterns in German.

You can use the same structure with many other nouns and verbs:

• Der Lehrer erklärt die Aufgabe.
• Die Ingenieurin testet das Gerät.
• Der Mechaniker repariert den Motor.

So this sentence is a good model for many other German sentences.

Yes. The noun itself is masculine, so its article changes depending on case:

• der Verdichter = nominative
• den Verdichter = accusative
• dem Verdichter = dative
• des Verdichters = genitive

Examples:

• Der Verdichter ist neu.
  The compressor is new.

• Der Techniker prüft den Verdichter.
  The technician checks the compressor.

• Der Techniker arbeitet mit dem Verdichter.
  The technician works with the compressor.

• Die Leistung des Verdichters ist hoch.
  The compressor’s performance is high.

So the article changes because of grammar, not because the noun changes gender.