Η φίλη μου ιδρώνει όταν περπατάει γρήγορα το μεσημέρι.

In Greek, the usual way to say my friend is η φίλη μου:

• η φίλη = the friend (feminine)
• μου = my

The possessive word μου normally comes after the noun, not before it as in English.

So:

• η φίλη μου = my friend
• literally: the friend of me

Greek often uses the definite article here, so η φίλη μου is the natural standard phrase.

Greek uses the definite article much more often than English. With family members, friends, body parts, and possessed nouns in general, Greek usually keeps the article.

So:

• η φίλη μου = my friend
• not usually just φίλη μου in a neutral sentence like this

The article also shows that φίλη is:

• feminine
• singular
• nominative (the subject of the sentence)

Ιδρώνει is the 3rd person singular present tense of ιδρώνω = to sweat.

So ιδρώνει means:

• she sweats
• he sweats
• it sweats

In this sentence, the subject is η φίλη μου, so here it means she sweats.

Greek verbs often include the subject inside the verb ending, which is why Greek does not need to say a separate word for she.

Greek is a pro-drop language, which means subject pronouns are often omitted when the verb ending already makes the subject clear.

So instead of saying:

• Η φίλη μου αυτή ιδρώνει or using a separate she

Greek simply says:

• Η φίλη μου ιδρώνει

The noun η φίλη μου is already the subject, and the verb ending -ει fits she/he/it.

Όταν means when and introduces a time clause.

So:

• όταν περπατάει γρήγορα = when she walks quickly

It tells you under what circumstances the sweating happens.

A useful pattern is:

• ιδρώνει όταν... = she sweats when...
• γελάει όταν... = she laughs when...
• κουράζεται όταν... = she gets tired when...

Yes. Περπατάει and περπατά are both common ways to write/say he/she walks or is walking in Modern Greek.

Both are correct. The longer form -άει is very common in everyday language.

So these are both natural:

• όταν περπατάει γρήγορα
• όταν περπατά γρήγορα

They mean the same thing here.

It can mean either, depending on context. Greek present tense often covers both the simple present and the present continuous.

So περπατάει can mean:

• walks
• is walking

In this sentence, English would most naturally translate it as something like:

• when she walks quickly
• or when she is walking quickly

The exact English choice depends on the style of translation, but the Greek form itself is normal for both.

Γρήγορα is an adverb, meaning quickly.

It modifies the verb περπατάει:

• περπατάει γρήγορα = walks quickly

Compare:

• γρήγορος = quick (masculine adjective)
• γρήγορη = quick (feminine adjective)
• γρήγορο = quick (neuter adjective)
• γρήγορα = quickly (adverb)

So here it is not describing a noun; it is describing how she walks.

Το μεσημέρι means at noon, at midday, or sometimes more broadly in the middle of the day / at lunchtime, depending on context.

Literally:

• το = the
• μεσημέρι = noon / midday

As a time expression, Greek often uses the article like this:

• το πρωί = in the morning
• το βράδυ = in the evening / at night
• το μεσημέρι = at noon / at midday

So το μεσημέρι is a very common set expression for time.

Many expressions of time in Greek use the accusative, especially fixed adverbial expressions.

So:

• το πρωί
• το απόγευμα
• το βράδυ
• το μεσημέρι

all function like adverbial time phrases, even though they look like article + noun.

You do not need to think of it as a direct object here. It is simply a standard way Greek expresses time when.

Not completely. Greek word order is more flexible than English, although some orders sound more neutral than others.

The given sentence:

• Η φίλη μου ιδρώνει όταν περπατάει γρήγορα το μεσημέρι.

is natural and neutral.

You could also hear:

• Η φίλη μου το μεσημέρι ιδρώνει όταν περπατάει γρήγορα.
• Όταν περπατάει γρήγορα το μεσημέρι, η φίλη μου ιδρώνει.

These shifts change emphasis more than basic meaning.

English usually depends more heavily on word order, but Greek can move parts around more freely.

Because φίλη is the feminine form of friend.

Compare:

• ο φίλος = the male friend / friend (masculine)
• η φίλη = the female friend (feminine)

So η φίλη μου clearly means my female friend.

If the sentence were about a male friend, it would be:

• Ο φίλος μου ιδρώνει όταν περπατάει γρήγορα το μεσημέρι.

The Greek present tense already works well for habitual actions, so the original sentence naturally suggests a general pattern:

• Η φίλη μου ιδρώνει όταν περπατάει γρήγορα το μεσημέρι.

That can mean something like:

• My friend sweats whenever she walks quickly at midday.

If you wanted to make the habitual meaning even clearer, you could add words such as:

• συνήθως = usually
• κάθε φορά που = every time that

For example:

• Η φίλη μου συνήθως ιδρώνει όταν περπατάει γρήγορα το μεσημέρι.
• Η φίλη μου ιδρώνει κάθε φορά που περπατάει γρήγορα το μεσημέρι.

A rough pronunciation guide is:

• ee FEE-lee moo ee-THRO-nee O-tan per-pa-TA-ee GREE-go-ra to me-see-ME-ri

A few key points:

• η sounds like ee
• δρ in ιδρώνει has the Greek δ, which sounds like the th in this
• γρήγορα begins with Greek γ, which is not exactly the English g; before front vowels it is softer
• stress matters a lot in Greek:
  • φίλη
  • ιδρώνει
  • περπατάει
  • γρήγορα
  • μεσημέρι

If you want to sound natural, pay close attention to the stressed syllables.