Η αδερφή μου πήρε καινούρια οδοντόβουρτσα, επειδή η παλιά δεν ήταν καλή πια.

Η αδερφή μου is the normal, neutral way to say my sister in Greek.

• η αδερφή = the sister
• μου = of me / my

So Greek often expresses possession as the X of me, not by putting a separate word before the noun the way English does.

You can sometimes hear η αδερφή μου and also more marked word orders in speech, but η αδερφή μου is the standard choice here.

μου here means my.

Grammatically, it is the weak genitive form of the pronoun εγώ. Greek uses these weak pronoun forms after nouns to show possession:

• ο αδερφός μου = my brother
• η αδερφή μου = my sister
• το σπίτι μου = my house

So even though we translate it as my, its grammar is closer to of me.

πήρε is the aorist of παίρνω and means took / got / bought, depending on context.

Here:

• παίρνω = I take / I get
• πήρε = he/she took / got

Because the subject is η αδερφή μου, it means:

• my sister got / bought

In this sentence, English would usually say bought or got, even though the basic Greek verb is literally take/get.

Greek can use παίρνω very naturally where English would say buy or get, especially in everyday speech.

So:

• πήρε καινούρια οδοντόβουρτσα literally = she got a new toothbrush
• but in context it often means she bought a new toothbrush

There is also a more specific verb for buy, αγοράζω, but παίρνω is extremely common in ordinary conversation.

Greek has no indefinite article equivalent to English a/an in many cases.

So:

• καινούρια οδοντόβουρτσα = a new toothbrush

Greek can add μια if it wants to emphasize one or make the noun feel more explicitly indefinite:

• πήρε μια καινούρια οδοντόβουρτσα

But it is very normal to leave it out, as in this sentence.

Because οδοντόβουρτσα is a feminine singular noun, the adjective has to agree with it.

• καινούριος = masculine
• καινούρια = feminine
• καινούριο = neuter

Since we have:

• η οδοντόβουρτσα = the toothbrush, feminine

the adjective must be feminine too:

• καινούρια οδοντόβουρτσα = new toothbrush

Yes, οδοντόβουρτσα is one word, meaning toothbrush.

It is a compound noun:

• οδόντ- / δόντι relates to tooth
• βούρτσα = brush

So it is literally something like tooth-brush, just as in English.

Yes, γιατί is often possible too.

Here, επειδή means because and introduces the reason:

• επειδή η παλιά δεν ήταν καλή πια = because the old one wasn’t good anymore

In many everyday contexts, επειδή and γιατί can both mean because.
However, γιατί can also mean why, so επειδή is sometimes clearer when you specifically want because.

In Greek, an adjective can be used substantively, meaning it stands in for a noun that is understood from context.

So:

• η παλιά literally = the old [one]

Here the omitted noun is οδοντόβουρτσα, so the full idea is:

• η παλιά οδοντόβουρτσα = the old toothbrush

But because it is obvious from context, Greek simply says:

• η παλιά = the old one

English does exactly the same thing in phrases like the old one.

Because the sentence is talking about a past situation.

• είναι = is
• ήταν = was

So:

• δεν ήταν καλή = it wasn’t good

Since the sister already got a new toothbrush, the sentence refers back to the condition of the old one in the past.

ήταν is the imperfect of είμαι.

Greek often uses the imperfect for a state, background description, or something that was true over a period of time in the past.

Here, the old toothbrush was not good anymore. That is a continuing condition, not a single completed action. So the imperfect fits naturally:

• δεν ήταν καλή πια = it wasn’t good anymore

καλή is feminine singular because it agrees with the understood noun οδοντόβουρτσα.

Even though the noun is omitted in the second clause, it is still understood:

• η παλιά [οδοντόβουρτσα] δεν ήταν καλή
• the old [toothbrush] wasn’t good

Since οδοντόβουρτσα is feminine, both adjectives are feminine:

• παλιά
• καλή

πια here means anymore / any longer / no longer.

So:

• δεν ήταν καλή πια = it wasn’t good anymore

This is a very common word in Greek. It often appears with negation:

• δεν θέλω πια = I don’t want [it] anymore
• δεν μένει εδώ πια = he/she doesn’t live here anymore

In a sentence like this, πια usually comes toward the end of the clause, often after the adjective or verb phrase it affects:

• δεν ήταν καλή πια

That is a very natural placement. Greek word order is somewhat flexible, but this position is the most straightforward and idiomatic here.

The comma separates the main clause from the reason clause:

• Η αδερφή μου πήρε καινούρια οδοντόβουρτσα
• επειδή η παλιά δεν ήταν καλή πια

In Greek, commas are often used before subordinate clauses like this, especially in clear written style. You will commonly see that punctuation with επειδή clauses.

A learner may also see αδελφή.

Both exist, but:

• αδερφή is very common in everyday Modern Greek
• αδελφή is more conservative/formal or associated with older spelling/style

So Η αδερφή μου is completely normal and natural in modern usage.

Yes, absolutely.

Both are natural:

• πήρε καινούρια οδοντόβουρτσα
• πήρε μια καινούρια οδοντόβουρτσα

Adding μια makes the noun phrase a bit more explicitly a certain/new one or simply more concrete, but the meaning stays basically the same here.

A very literal version would be:

• The sister my took new toothbrush, because the old [one] not was good anymore.

That sounds unnatural in English, but it helps show the Greek structure:

• Η αδερφή μου = the sister my
• πήρε = took/got
• καινούρια οδοντόβουρτσα = new toothbrush
• επειδή = because
• η παλιά = the old one
• δεν ήταν καλή πια = was not good anymore