Ο αστράγαλός μου πονάει λίγο όταν περπατάω πολύ.

Greek very often uses the definite article with body parts and with possessive expressions.

So:

• ο αστράγαλός μου = my ankle
• literally: the ankle of me / my ankle

This is normal Greek structure. English usually does not use the here, but Greek usually does.

This happens because μου is an enclitic word, meaning it leans onto the previous word.

The basic noun is:

• ο αστράγαλος

But before enclitics like μου, σου, του, Greek often adds an extra written accent if the noun is stressed on the third syllable from the end.

So:

• ο αστράγαλος
• ο αστράγαλός μου

This does not mean it is a different noun or a different case. It is mainly an accentuation rule.

μου means my here.

More literally, it is the weak form of to me / of me, but in expressions like this it functions as a possessive:

• ο αστράγαλός μου = my ankle
• το χέρι μου = my hand
• η πλάτη μου = my back

So Greek often says the ankle my, where English says my ankle.

Because the subject is singular:

• Ο αστράγαλός μου = my ankle

So the verb must also be singular:

• πονάει = hurts

If the subject were plural, the verb would change:

• Οι αστράγαλοί μου πονάνε = My ankles hurt

These are related forms of the same verb, πονάω / πονώ = to hurt / to be in pain.

Common present-tense forms include:

• πονάω
• πονάς
• πονάει / πονά
• πονάνε / πονούν

So πονάει and πονά are both possible for he/she/it hurts.
In everyday speech, πονάει is very common.

Here:

• Ο αστράγαλός μου πονάει = My ankle hurts

λίγο means a little or slightly.

In this sentence it works as an adverb modifying the verb:

• πονάει λίγο = hurts a little

Compare:

• Πεινάω λίγο. = I’m a little hungry.
• Κουράστηκα λίγο. = I got a little tired.

So it is not little as an adjective here; it is a little / slightly.

That word order is very natural in Greek:

• πονάει λίγο = hurts a little

Greek word order is somewhat flexible, but verb + adverb is very common.
You may also hear other orders for emphasis, but this version sounds neutral and natural.

όταν means when.

In this sentence, it introduces something that happens repeatedly / habitually:

• when I walk a lot

Greek often uses the present tense for this kind of general repeated situation:

• όταν περπατάω πολύ = when I walk a lot

This does not mean only right now. It can mean whenever I walk a lot.

Both are acceptable forms of the verb to walk.

• περπατάω
• περπατώ

In everyday Modern Greek, forms in -άω are very common and often sound more natural in casual speech.

So:

• όταν περπατάω πολύ and
• όταν περπατώ πολύ

both mean the same thing.

Because Greek usually does not need subject pronouns when the verb ending already shows who the subject is.

• περπατάω = I walk

The ending tells you it is I, so εγώ is usually omitted unless you want emphasis or contrast.

So:

• όταν περπατάω πολύ = when I walk a lot
• όταν εγώ περπατάω πολύ would sound more emphatic, like when I walk a lot (not someone else)

Here πολύ means a lot.

It modifies περπατάω:

• περπατάω πολύ = I walk a lot

So the phrase means:

• when I walk a lot or
• when I do a lot of walking

Do not confuse this with πολύς, πολλή, πολύ as an adjective meaning much/many. Here it is being used adverbially.

Yes. Greek word order is more flexible than English.

The given sentence:

• Ο αστράγαλός μου πονάει λίγο όταν περπατάω πολύ.

is neutral and natural.

But Greek could also say things like:

• Όταν περπατάω πολύ, ο αστράγαλός μου πονάει λίγο.

This puts the when clause first and may sound slightly more organized or emphatic in context.

The meaning stays basically the same.

Here it means it hurts / is painful.

The subject is the ankle itself:

• Ο αστράγαλός μου πονάει = My ankle hurts

Greek can also use πονάω in other patterns, for example:

• Πονάω το χέρι μου. = My hand hurts.
• Με πονάει το χέρι μου. = My hand hurts me / more literally My hand is hurting me

But in your sentence, the simple intransitive sense is being used:

• my ankle hurts

Approximately like this:

• O a-STRA-ga-LOS moo po-NA-i LEE-go O-tan per-pa-TA-o po-LEE

A few helpful notes:

• στρ in αστράγαλός is a consonant cluster, so say it smoothly.
• αι in πονάει sounds like e in Modern Greek pronunciation.
• ω and ο sound the same in Modern Greek.
• Stress matters a lot:
  • αστραγαλός
  • πονάει
  • λίγο
  • περπατάω
  • πολύ

Yes. That is a very natural English rendering.

The Greek πολύ literally means a lot, but in context English may translate it as:

• a lot
• too much
• for a long time
• a lot of walking

depending on the tone and situation.

So natural translations include:

• My ankle hurts a little when I walk a lot.
• My ankle hurts a bit when I do a lot of walking.
• My ankle hurts a bit when I walk too much.