Η καρδιά μου ηρεμεί όταν περπατάω αργά στο πάρκο.

Η καρδιά μου means my heart.

• η = the for a feminine singular noun
• καρδιά = heart
• μου = my

So Greek literally says the heart my, where English says my heart. This is a very common Greek pattern with possessive pronouns.

In Greek, possessive words like μου (my), σου (your), του/της (his/her) often come after the noun:

• η καρδιά μου = my heart
• το βιβλίο σου = your book
• ο φίλος της = her friend

This is the normal way to express possession in everyday Greek.

Because the subject of that verb is η καρδιά μου (my heart), which is third person singular.

So:

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

Since heart is the thing doing the action grammatically, Greek uses ηρεμεί.

ηρεμεί is the present tense, 3rd person singular, from the verb ηρεμώ or ηρεμώ/ηρεμώ? More commonly you will learn it as ηρεμώ = to calm down / to become calm.

Here it expresses a present or habitual idea:

• Η καρδιά μου ηρεμεί = My heart calms down / My heart is calmed

In this sentence it most naturally has a general, repeated meaning: this happens when I walk slowly in the park.

όταν means when.

It introduces a time clause:

• όταν περπατάω αργά στο πάρκο = when I walk slowly in the park

In sentences like this, όταν is very commonly used to describe something that happens whenever another thing happens.

Because the subject changes.

• In the main clause, the subject is η καρδιά μου → so the verb is ηρεμεί
• In the όταν clause, the implied subject is I → so the verb is περπατάω

So Greek is saying:

• My heart calms down
• when I walk slowly in the park

Greek often leaves out the subject pronoun εγώ (I) because the verb ending already shows who the subject is.

Yes. Both are correct.

• περπατάω
• περπατώ

Both mean I walk. The -άω and -ώ forms are common alternative present forms for some verbs in Modern Greek.

So you may see:

• όταν περπατάω
• όταν περπατώ

Both are natural, though περπατάω is often felt as especially common in everyday speech.

αργά means slowly here. It is an adverb, modifying the verb περπατάω.

So:

• περπατάω αργά = I walk slowly

Compare:

• αργός = slow for a masculine noun
• αργή = slow for a feminine noun
• αργό = slow for a neuter noun
• αργά = slowly as an adverb

Because στο is the contracted form of σε + το.

• σε = in / to / at
• το = the for a neuter singular noun
• στο = in the / to the / at the

So:

• στο πάρκο = in the park

This contraction is standard and very common:

• στο σπίτι = in the house / at home
• στην πόλη = in the city
• στον δρόμο = on the street

Greek nouns have grammatical gender, and πάρκο happens to be neuter.

You can tell from the article:

• το πάρκο = the park

Many neuter nouns end in -ο, though not all nouns ending in -ο behave exactly the same way. Here you simply learn το πάρκο as the dictionary form.

Greek word order is fairly flexible. This sentence could also be reordered, for example:

• Όταν περπατάω αργά στο πάρκο, η καρδιά μου ηρεμεί.

That means the same thing: My heart calms down when I walk slowly in the park.

The original order is perfectly natural. Changing the order usually changes emphasis, not the basic meaning.

It can be both.

• literally: the physical heart
• figuratively: the emotional heart, feelings, inner self

In this sentence, Η καρδιά μου ηρεμεί sounds natural in a slightly emotional or poetic way, like my heart becomes calm or I feel inwardly calm.

Most naturally, it describes a general/habitual truth:

• My heart calms down when I walk slowly in the park

The Greek present tense can often express:

• something happening now
• something that happens regularly
• a general pattern

Without extra context, this sentence sounds like a general repeated experience.

Greek often drops subject pronouns because the verb ending already makes the subject clear.

• περπατάω already means I walk

So Greek does not need εγώ unless you want special emphasis:

• όταν εγώ περπατάω... would sound more emphatic, like when I walk...

In ordinary speech, leaving εγώ out is normal.

A simple pronunciation guide is:

i kar-DHYA mu i-re-MI o-tan per-pa-TA-o ar-GHA sto PAR-ko

A few useful notes:

• η, ι, υ, ει, οι often sound like English ee in see
• δ sounds like th in this
• γ before α, ο, ου is a voiced sound somewhat like a soft gargled gh
• stress matters a lot in Greek, so pay attention to:
  • καρδιά
  • ηρεμεί
  • περπατάω
  • αργά
  • πάρκο

Yes. That is also possible.

Greek can place the verb first for stylistic or emphasis reasons:

• Η καρδιά μου ηρεμεί όταν περπατάω αργά στο πάρκο
• Ηρεμεί η καρδιά μου όταν περπατάω αργά στο πάρκο

Both are understandable and natural. The first version is a very straightforward neutral order.