Mein Arm blutet.

Because Arm is a masculine noun in German: der Arm.

The possessive word mein changes depending on the gender, number, and case of the noun:

• mein Arm = my arm
• meine Hand = my hand
• mein Bein = my leg

So here, mein is the correct form because Arm is masculine singular and is the subject of the sentence.

In German, all nouns are capitalized, not just proper names.

So:

• mein Arm
• das Haus
• eine Katze

This is one of the most noticeable spelling differences between English and German.

Blutet is the 3rd person singular form of the verb bluten, which means to bleed.

The subject is mein Arm, which is grammatically he/she/it in English terms, so German uses the er/sie/es form:

• ich blute = I bleed / am bleeding
• du blutest = you bleed / are bleeding
• er/sie/es blutet = he/she/it bleeds / is bleeding

So Mein Arm blutet. literally works like My arm bleeds or more naturally My arm is bleeding.

In German, the simple present tense often covers both:

• My arm bleeds
• My arm is bleeding

So Mein Arm blutet can mean either, depending on context.

German does not usually need a special progressive form like English is bleeding. The normal present tense is enough.

Because mein already works like a determiner here. It fills the same slot that der would fill.

Compare:

• der Arm = the arm
• mein Arm = my arm

You normally do not say mein der Arm. In standard German, it is either the article or the possessive word, not both together.

This is the normal German main clause word order:

subject + verb

So:

• Mein Arm = subject
• blutet = verb

German main clauses usually put the finite verb in second position. Since the subject comes first here, the verb comes right after it.

You could change the order for emphasis:

• Mein Arm blutet. = neutral
• Blutet mein Arm? = Is my arm bleeding?
• Jetzt blutet mein Arm. = Now my arm is bleeding.

Yes. Bluten is a regular verb.

Its present tense forms are:

• ich blute
• du blutest
• er/sie/es blutet
• wir bluten
• ihr blutet
• sie/Sie bluten

So blutet follows the normal pattern for a regular verb.

A simple English-friendly guide would be:

• mein ≈ mine
• Arm ≈ ahrm
• blutet ≈ BLOO-tet

A bit more accurately:

• mein has the same kind of sound as English mine
• Arm has a broad a sound
• blutet has stress on the first syllable: BLU-tet

So the whole sentence sounds roughly like:

mine ahrm BLOO-tet

Yes, very often German uses a definite article with body parts, especially when possession is clear from context.

For example:

• Der Arm blutet. = The arm is bleeding.
• Mir blutet der Arm. = My arm is bleeding / My arm is bleeding on me.

But Mein Arm blutet is also completely natural, especially when you want to be explicit.

So both patterns exist:

• mein Arm = explicit possession
• der Arm = possession understood from context

They are both possible, but they focus on slightly different things.

• Mein Arm blutet. = The arm itself is the focus.
• Ich blute am Arm. = I am bleeding on/in the arm area.

If you are describing an injured body part directly, Mein Arm blutet is very natural and straightforward.

No. It can also be used for wounds, noses, and even figuratively.

Examples:

• Die Wunde blutet. = The wound is bleeding.
• Meine Nase blutet. = My nose is bleeding.
• Die Firma blutet Geld. = The company is bleeding money.

So in Mein Arm blutet, it is being used in a very literal, physical sense.

It is in the nominative case because it is the subject of the sentence.

You can test this by asking: What is bleeding? Answer: mein Arm

That makes it the subject, so nominative is used.

• mein Arm = nominative
• if it were an object in another sentence, the form might change:
  • Ich bewege meinen Arm. = I move my arm.
    Here meinen Arm is accusative.

So in Mein Arm blutet, nominative is correct.