Η φίλη μου ηρεμεί όταν ακούει ήρεμη μουσική.

In Greek, possessive phrases normally use the definite article:

• η φίλη μου = my friend
• το σπίτι μου = my house
• ο αδερφός της = her brother

So η here is not extra in meaning; it is just the normal Greek way to say my friend. Because φίλη is feminine singular nominative, the article is η.

Greek unstressed possessives like μου, σου, του, της, μας, σας, τους usually come after the noun:

• η φίλη μου = my friend
• το βιβλίο σου = your book

This is different from English, where my comes before the noun. So η φίλη μου is the standard order.

It can mean either, depending on context.

• φίλη literally means a female friend
• In many contexts, η φίλη μου can also mean my girlfriend

Only context tells you which one is meant. If the sentence is just for grammar practice, it is usually safest to think of it as my friend unless the broader context suggests a romantic meaning.

Greek often drops subject pronouns because the verb ending already shows the person and number.

• ηρεμεί = he/she/it calms down or is calming down
• ακούει = he/she/it listens / hears

Because the subject is already clear from η φίλη μου, Greek does not need to repeat αυτή (she). Adding αυτή would usually give extra emphasis.

Both are in the present tense.

• ηρεμεί = present, 3rd person singular
• ακούει = present, 3rd person singular

In this sentence, the present tense expresses a general or habitual idea: this is what happens when she listens to calm music. It is not necessarily only happening right now.

They are related, but they are different parts of speech.

• ηρεμεί is a verb: (she) calms down / becomes calm
• ήρεμη is an adjective: calm

So:

• Η φίλη μου ηρεμεί = My friend calms down
• ήρεμη μουσική = calm music

They come from the same idea of calmness, but one is an action/state as a verb, and the other is a descriptive adjective.

The adjective has to agree with the noun it describes in gender, number, and case.

Here the noun is:

• μουσική = feminine singular

So the adjective must also be feminine singular:

• masculine: ήρεμος
• feminine: ήρεμη
• neuter: ήρεμο

That is why Greek uses ήρεμη μουσική.

It is the direct object of ακούει, so grammatically it is in the accusative case.

However, with many feminine nouns like μουσική, the accusative singular looks the same as the nominative singular:

• nominative: η μουσική
• accusative: τη μουσική

Without the article, the noun form itself stays μουσική, so you do not see a visible ending change here. The function is accusative, even though the form looks the same.

Because Greek often omits the article when speaking generally about something, especially with uncountable nouns or when the noun is being used in a broad sense.

So:

• ακούει ήρεμη μουσική = she listens to calm music

This sounds like calm music in general, not a specific known piece or set of music.

If you wanted to refer to specific calm music, you could say:

• ακούει την ήρεμη μουσική = she listens to the calm music

That would sound more specific.

It can mean both, depending on context.

In Modern Greek, ακούω covers both ideas:

• hear
• listen to

In this sentence, because the object is ήρεμη μουσική, the natural English meaning is listens to calm music, not just hears calm music.

Because Greek grammar works differently from English here. The verb ακούω takes a direct object:

• ακούω μουσική = I listen to music / I hear music

English needs the preposition to with listen, but Greek does not. So μουσική comes directly after ακούει.

όταν means when and introduces a time clause.

So:

• όταν ακούει ήρεμη μουσική = when she listens to calm music

In sentences like this, Greek commonly uses the present tense after όταν when talking about repeated or general situations.

Yes. Greek word order is more flexible than English, although some orders sound more neutral than others.

The original order is very natural:

• Η φίλη μου ηρεμεί όταν ακούει ήρεμη μουσική.

You could also say:

• Όταν ακούει ήρεμη μουσική, η φίλη μου ηρεμεί.

This means the same thing, but now the when clause is placed first for focus or style.

Because they are different word forms, and Greek stress is part of each form.

• ηρεμεί has the stress on the last syllable
• ήρεμη has the stress on the first syllable

You should learn the stress together with each form, because stress in Greek is important for correct pronunciation and sometimes helps distinguish related forms.