Der Arzt kontrolliert meinen Arm.

Because der Arzt is the subject of the sentence — the person doing the action.

In German, the masculine definite article changes by case:

• der = masculine nominative (subject)
• den = masculine accusative (direct object)

Here, the doctor is the one who checks, so German uses der Arzt.

Because Arm is the direct object here, and it is masculine.

In German, a masculine noun in the accusative case changes the article or possessive determiner:

• mein Arm = nominative
• meinen Arm = accusative

So in this sentence:

• Der Arzt = subject
• kontrolliert = verb
• meinen Arm = direct object

That is why mein becomes meinen.

You usually learn German nouns together with their article:

• der Arm = arm

Since the dictionary form is der Arm, the noun is masculine.

That matters because gender affects:

• articles
• adjective endings
• possessive words like mein/meinen

So it is best to memorize nouns as a package, for example:

• der Arm
• der Arzt

not just Arm or Arzt by themselves.

It is in the accusative case.

The accusative is commonly used for the direct object — the thing directly affected by the action.

In Der Arzt kontrolliert meinen Arm:

• Who is doing the action? Der Arzt
• What is being checked? meinen Arm

So meinen Arm is the direct object, and therefore accusative.

Because the subject is der Arzt, which is third person singular: he/she/it.

The verb is kontrollieren. Its present-tense forms are:

• ich kontrolliere
• du kontrollierst
• er/sie/es kontrolliert
• wir kontrollieren
• ihr kontrolliert
• sie/Sie kontrollieren

Since der Arzt corresponds to er, the correct form is kontrolliert.

No — and this is an important point.

In this sentence, kontrollieren usually means to check, to examine, or to inspect, not to control in the English sense of to dominate or to manage.

So although the words look similar, they are not always used the same way. This is a classic example of a false friend or at least a partly misleading cognate.

In a medical context, Der Arzt kontrolliert meinen Arm usually means the doctor is checking/examining your arm.

Because German main clauses normally follow the verb-second rule.

That means:

• one element comes first
• the finite verb comes second

Here:

1. Der Arzt
2. kontrolliert
3. meinen Arm

This is the normal, straightforward word order for a simple statement: Subject + verb + object

German can sometimes change word order for emphasis, but this version is the standard neutral one.

Because all nouns are capitalized in German.

So:

• der Arzt
• mein Arm

This is a regular rule in German, unlike English, where common nouns are usually lowercase.

Because meinen Arm is accusative masculine, not dative.

For the possessive mein-, the masculine forms are:

• mein = nominative masculine
• meinen = accusative masculine
• meinem = dative masculine

So:

• mein Arm = my arm (subject form)
• meinen Arm = my arm (direct object form)
• meinem Arm = to my arm / on my arm, depending on context

In this sentence, the arm is the direct object, so meinen is correct.

Grammatically, der Arzt is the masculine form.

Traditionally:

• der Arzt = male doctor
• die Ärztin = female doctor

In older or more general usage, the masculine form could sometimes be used generically, but in modern German people are often more careful about gender-specific wording.

So if the doctor is known to be female, you would normally say: Die Ärztin kontrolliert meinen Arm.

A rough English-friendly guide is:

• Arzt ≈ ahtst

A few details:

• the A is like a broad ah
• the r is usually not strongly pronounced like an English r
• zt at the end sounds like tst

So it may feel a little tricky at first because of the final consonant cluster.

Sometimes, yes, but Arzt is usually the more neutral and common word for a medical doctor in everyday German.

• der Arzt = doctor / physician
• der Doktor = someone with a doctorate; it can also be used for a doctor, but it is not always the best everyday equivalent

So if you specifically mean a medical professional, Arzt is usually the safest choice.

Yes, grammatically you could, but the meaning changes slightly.

• meinen Arm = my arm
• den Arm = the arm

If you want to say whose arm it is, German uses the possessive: meinen Arm

If the context already makes it obvious whose arm it is, German sometimes uses a definite article in places where English prefers a possessive, especially with body parts. But in a simple learner sentence, meinen Arm is very natural and clear.

Because in this sentence, the case marking is shown mainly by the determiner meinen, not by the noun Arm.

German often marks case through:

• articles
• possessives
• adjectives

The noun itself does not always change.

So:

• der Arm
• meinen Arm

The noun Arm stays the same here, while mein changes to meinen.

The dictionary forms are:

• der Arzt = doctor
• kontrollieren = to check / examine / inspect
• der Arm = arm
• mein = my

When you learn German, it helps to store:

• verbs in the infinitive, like kontrollieren
• nouns with their article, like der Arzt, der Arm

That makes it much easier to build correct sentences later.