Η φίλη μου περπατάει αργά στην ανηφόρα γιατί πονάει λίγο το γόνατό της.

Greek often uses the definite article with possessed nouns, where English usually does not.

So η φίλη μου literally looks like the friend my, but it means my friend.

Using the article here is completely normal Greek. Without η, the phrase could sound less neutral or more context-dependent.

In Greek, weak possessive forms like μου, σου, της, μας usually come after the noun.

So:

• η φίλη μου = my friend
• το σπίτι σου = your house
• η μητέρα της = her mother

This is one of the big differences from English word order.

Greek often leaves out subject pronouns when they are already clear from the verb form. This is called a pro-drop language.

So instead of saying Αυτή περπατάει, Greek normally just says περπατάει, because the ending already tells you it is he/she/it walks.

In this sentence, η φίλη μου has already been mentioned, so there is no need to add αυτή.

Yes. Both mean he/she/it walks or is walking.

For many verbs in Modern Greek, especially in everyday language, you may see two common 3rd person singular present forms:

• περπατάει
• περπατά

Both are correct. περπατάει can sound a bit fuller or slightly more conversational in some contexts, but both are standard.

Αργά is an adverb, and here it means slowly.

It describes how she walks:

• περπατάει αργά = she walks slowly

Do not confuse this with the sense late, which αργά can also have in other contexts. Here, because it goes with walking, it clearly means slowly.

Στην is the contracted form of σε την.

So:

• σε + την = στην
• σε + το = στο

This contraction is extremely common and usually preferred in normal speech and writing.

So στην ανηφόρα means on the uphill road / on the incline / uphill.

It is in the accusative.

The preposition σε normally takes the accusative in Modern Greek. Since στην comes from σε την, the noun after it is accusative:

• nominative: η ανηφόρα
• accusative: την ανηφόρα

So in the sentence, στην ανηφόρα is the expected form after σε.

Yes, γιατί can mean both because and why in Modern Greek.

You tell the difference from context:

• Γιατί περπατάει αργά; = Why is she walking slowly?
• Περπατάει αργά γιατί πονάει... = She walks slowly because it hurts...

In your sentence, γιατί introduces an explanation, so it means because.

The subject is το γόνατό της.

So the structure is basically:

• πονάει λίγο το γόνατό της
• her knee hurts a little

In Greek, the subject does not always have to come before the verb. It can come after it, especially in natural narration.

So even though το γόνατό της comes later, it is still the subject, and that is why πονάει is singular.

Greek word order is more flexible than English word order.

Both of these are possible:

• πονάει λίγο το γόνατό της
• το γόνατό της πονάει λίγο

The version in your sentence sounds natural and gives a smooth reason/explanation after γιατί. Putting the verb first can make the clause sound more flowing or slightly more focused on the action/state.

So this is not a different grammar rule so much as a natural Greek word-order choice.

This happens because της is an enclitic word.

The noun γόνατο normally has its stress on the third-to-last syllable:

• γόνατο

When an enclitic such as μου, σου, της, του follows a word like that, Greek spelling often adds a second accent:

• το γόνατό της

This extra accent helps show the correct pronunciation pattern.

Compare:

• το πρόσωπό μου
• το όνομά της
• το γόνατό της

But not every noun gets this extra accent. It happens especially when the main word is stressed far enough from the end that the enclitic would otherwise disturb the normal stress pattern.

Greek usually keeps the definite article with possessed nouns, especially with body parts.

So Greek naturally says:

• το γόνατό της = her knee
• το χέρι μου = my hand
• τα μάτια του = his eyes

This is different from English, where the possessive usually replaces the article.

Here λίγο means a little or slightly.

It modifies πονάει, not γόνατο. So the idea is:

• it hurts a little
• her knee hurts a bit

It is functioning adverbially, describing the degree of pain.

Not very naturally. A very literal breakdown would be something like:

• Η φίλη μου = the friend my
• περπατάει αργά = walks slowly
• στην ανηφόρα = on the uphill / up the slope
• γιατί = because
• πονάει λίγο = hurts a little
• το γόνατό της = the knee her

But Greek and English organize possession, word order, and articles differently. So a natural English translation needs to be less literal.

That is very common when learning Greek: the meaning is straightforward, but the structure is often different from English.