Όταν ακούω ήρεμη μουσική, η καρδιά μου ηρεμεί και κοιμάμαι καλύτερα.

In this sentence, Όταν means when, but in context it often has the broader sense of whenever.

Because the whole sentence is in the present tense, it describes a general, repeated situation:

• Όταν ακούω ήρεμη μουσική... = When / Whenever I listen to calm music...

So Greek Όταν can introduce:

• a specific time: When I arrive, call me
• a habitual situation: Whenever I drink coffee, I feel better

Here it is the habitual meaning.

Greek uses the present tense here for a general truth or regular experience, just like English often does.

So the sentence means something like:

• Whenever I listen to calm music, my heart calms down and I sleep better.

It is not talking about only one specific moment. It describes what usually happens.

The verbs are:

• ακούω = I listen / I hear
• ηρεμεί = it calms down
• κοιμάμαι = I sleep

ήρεμη is the adjective calm, and it agrees with μουσική in gender, number, and case.

The noun μουσική is:

• feminine
• singular
• accusative, because it is the direct object of ακούω

So the adjective must match it:

• base form: ήρεμος = calm
• feminine form: ήρεμη
• neuter form: ήρεμο

So:

• ήρεμη μουσική = calm music

A useful point: for many feminine nouns and adjectives in -η, the nominative and accusative singular look the same, so you do not see a visible change here.

Greek often leaves out the article when speaking about something in a general or indefinite way.

So ακούω ήρεμη μουσική means:

• I listen to calm music
• not necessarily the calm music
• not necessarily some calm music in a strongly marked way

If you added an article, the meaning would become more specific:

• ακούω την ήρεμη μουσική = I listen to the calm music
  meaning some specific music already known in the conversation

Without the article, it sounds natural and general.

In Greek, possessive words like μου usually come after the noun.

So:

• η καρδιά μου = my heart
• literally: the heart my

This is the normal Greek pattern:

• το βιβλίο μου = my book
• η φίλη μου = my friend
• το σπίτι μου = my house

Notice that Greek usually keeps the article:

• η καρδιά μου not
• καρδιά μου in neutral standard usage here

ηρεμεί is the 3rd person singular present form of ηρεμώ, which means to calm down or to become calm.

So:

• ηρεμώ = I calm down
• ηρεμεί = he/she/it calms down

In this sentence, the subject is:

• η καρδιά μου = my heart

So ηρεμεί means:

• my heart calms down

Even though καρδιά is grammatically feminine, the English translation uses calms down without needing a gendered pronoun.

It goes back to I.

This is a very common thing to notice in Greek. The sentence has two different subjects:

• η καρδιά μου ηρεμεί = my heart calms down
• κοιμάμαι καλύτερα = I sleep better

How do we know? From the verb form:

• ηρεμεί = 3rd person singular = it / she / he calms down
• κοιμάμαι = 1st person singular = I sleep

So even though there is no written εγώ, the verb ending tells you the subject has changed back to I.

That is because κοιμάμαι is one of the many Modern Greek verbs that use middle/passive-style endings but have an active meaning.

So:

• κοιμάμαι = I sleep
• not I am slept

This is normal Greek grammar. Many common verbs work this way.

The form belongs to the -μαι pattern:

• κοιμάμαι = I sleep
• κοιμάσαι = you sleep
• κοιμάται = he/she sleeps

So even though it looks passive to an English speaker, here it is simply the normal verb for sleep.

Because καλύτερα is an adverb, and it modifies the verb κοιμάμαι.

It answers the question:

• How do I sleep?
  Better.

So:

• καλός / καλή / καλό = good as an adjective
• καλά = well as an adverb
• καλύτερα = better as an adverb

That is why Greek says:

• κοιμάμαι καλύτερα = I sleep better

If you used καλύτερος, that would be an adjective meaning better for a noun, not for a verb.

Because Όταν ακούω ήρεμη μουσική is an introductory subordinate clause, and Greek normally separates it from the main clause with a comma.

So the structure is:

• Όταν ακούω ήρεμη μουσική, = subordinate clause
• η καρδιά μου ηρεμεί και κοιμάμαι καλύτερα. = main clause

This is similar to English:

• When I listen to calm music, my heart calms down and I sleep better.

The comma helps show the boundary between the two parts.

Because Greek often leaves out subject pronouns when the verb ending already makes the subject clear.

So:

• ακούω already means I listen
• κοιμάμαι already means I sleep

Greek is a pro-drop language, which means subject pronouns are often omitted unless needed for emphasis or contrast.

So:

• ακούω ήρεμη μουσική = I listen to calm music
• εγώ ακούω ήρεμη μουσική would mean something more like I listen to calm music, possibly with extra emphasis

In this sentence, no emphasis is needed, so Greek naturally leaves εγώ out.

It can cover both meanings depending on context.

In this sentence, ακούω ήρεμη μουσική is best understood as:

• I listen to calm music

But Greek ακούω can also mean:

• I hear

For example:

• Ακούω έναν θόρυβο. = I hear a noise.

So the verb is broader than English listen. Context tells you whether it means hear or listen to. Here, with music, the meaning is naturally listen to.