Der Mechaniker klopft auf den Reifen.

Mechaniker is a masculine noun, and here it is the subject of the sentence, so it takes the nominative definite article der.

• der Mechaniker = the mechanic
• If it were the direct object, you would often see den Mechaniker

So Der Mechaniker simply means the mechanic as the person doing the action.

Klopft is the 3rd person singular form of the verb klopfen.

The subject is Der Mechaniker, which is equivalent to he in English, so German uses the er/sie/es form:

• ich klopfe
• du klopfst
• er/sie/es klopft

So:

• Der Mechaniker klopft = The mechanic knocks / is knocking

With klopfen auf, German normally uses the accusative to show the thing being knocked on:

• auf den Reifen klopfen = to knock on the tire

Here, Reifen is masculine, and the masculine accusative article is den.

So:

• der Reifen = nominative
• den Reifen = accusative

A helpful way to think about it is that auf here is part of the expression auf etwas klopfen = to knock on something.

Den Reifen is accusative singular masculine.

You can tell because:

1. Reifen is a masculine noun: der Reifen
2. The article changes from der to den in the accusative
3. The phrase auf den Reifen follows the common pattern auf + accusative with klopfen

So the form den is the main clue that this is accusative.

It can mean either one, depending on context.

German uses the simple present tense much more broadly than English does. So:

• Der Mechaniker klopft auf den Reifen. can mean
  • The mechanic knocks on the tire.
  • The mechanic is knocking on the tire.

English often chooses between simple present and present progressive, but German usually just uses the present tense.

In a normal German main clause, the finite verb usually comes in second position. This is called the V2 rule.

In this sentence:

• Der Mechaniker = first element
• klopft = second element
• auf den Reifen = the rest

So the structure is:

• Subject + verb + other information

That is why klopft comes right after Der Mechaniker.

Here, klopfen means to knock or to tap.

It usually suggests a light repeated hitting motion, not a hard strike. In this sentence, it sounds like the mechanic is tapping the tire, perhaps to check it.

So klopfen is softer than verbs like:

• schlagen = to hit
• stoßen = to shove / bump

This is a very common question.

• der Reifen = tire
• das Rad = wheel

A Reifen is the rubber outer part. A Rad can mean the whole wheel unit.

So in this sentence, Reifen is the natural word if the mechanic is specifically knocking on the tire.

A learner-friendly pronunciation is roughly:

• klopft ≈ KLOPFt

The tricky part is the final consonant cluster -pft. German allows these tight clusters, so the ending can feel difficult at first.

You can build it step by step:

• klopf
• then add a light t

Also remember:

• kl as in klopfen
• o is short here
• pf is a real combination in German, pronounced with both sounds

Because all German nouns are capitalized.

So in this sentence:

• Mechaniker is a noun
• Reifen is a noun

That is why both begin with capital letters. This is a standard rule in German and helps you spot nouns quickly when reading.