Η αδερφή μου φόρεσε άλλα παπούτσια, γιατί πονούσε η φτέρνα της.

η is the feminine singular definite article, so it means the.

In this sentence, η αδερφή μου literally means the sister of mine, which is the normal Greek way to say my sister. The whole phrase is the subject of the sentence.

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

So:

• η αδερφή μου = my sister
• το σπίτι μου = my house
• οι φίλοι μου = my friends

Literally, μου is related to of me, but in normal English translation it becomes my.

Yes. Both mean sister.

αδερφή is very common in everyday Modern Greek, while αδελφή is a more conservative or traditional spelling/form. A learner will hear and see both, but αδερφή is extremely common in speech.

φόρεσε is the aorist form, third person singular, of φοράω / φορώ.

Here it describes a completed action: she put on or wore different shoes. In context, English might translate it as she put on other shoes or she changed into different shoes.

Compare:

• φόρεσε = she put on / wore once, as a complete action
• φορούσε = she was wearing

άλλα is the neuter plural form of άλλος, meaning other or different.

It agrees with παπούτσια, which is a neuter plural noun:

• άλλος = masculine singular
• άλλη = feminine singular
• άλλο = neuter singular
• άλλα = neuter plural

So άλλα παπούτσια means other shoes or different shoes.

No. They are different words.

• άλλα = other / different
• αλλά = but

The accent changes the meaning. In this sentence, άλλα παπούτσια clearly means different shoes, not but shoes.

παπούτσια is a neuter plural noun, and in Greek many neuter nouns have the same form in the nominative and accusative.

So:

• nominative plural: παπούτσια
• accusative plural: παπούτσια

That is why after φόρεσε it still appears as παπούτσια. The singular is παπούτσι.

Here γιατί means because.

Greek uses γιατί for both:

• why?
• because

You understand which meaning it has from the sentence structure and context. In this sentence, it introduces a reason: because her heel was hurting.

πονούσε is the imperfect, which shows an ongoing or background situation in the past.

So πονούσε η φτέρνα της means her heel was hurting or her heel hurt in an ongoing sense. That fits well as the reason she changed shoes.

If you used πόνεσε instead, it would sound more like it hurt at one moment or it started to hurt, which gives a different nuance.

That is a very normal Greek structure.

Greek often uses:

• the definite article
• the noun
• a weak genitive pronoun

So:

• η φτέρνα της = literally the heel of her
• natural English: her heel

This is especially common with body parts and personal belongings.

Because Greek is saying her heel was hurting.

So grammatically:

• πονούσε = was hurting
• η φτέρνα της = her heel

This is actually similar to a natural English structure: her heel was hurting. Greek simply uses that pattern directly.

Yes. That would also be correct.

Greek word order is more flexible than English. Both of these are possible:

• γιατί πονούσε η φτέρνα της
• γιατί η φτέρνα της πονούσε

The version in your sentence is very natural. Putting the verb before the subject is common in Greek, especially in flowing narration.