Η ταυτότητά σου δεν είναι πια έγκυρη, γιατί έχει λήξει και πρέπει να την ανανεώσεις.

Because ταυτότητα is a feminine singular noun, so in the nominative it normally appears as η ταυτότητα.

Greek very often keeps the definite article even with possession:

• η ταυτότητά σου = your ID
• literally, something like the ID of yours

So the article is completely normal here.

This is because σου is an enclitic, a little unstressed word that leans on the word before it.

When a word like ταυτότητα is stressed on the antepenultimate syllable and is followed by an enclitic such as σου, Greek adds an extra written accent:

• ταυτότητα
• ταυτότητά σου

So the extra accent is not changing the basic word; it is showing the correct stress pattern when σου follows.

In Greek, the common unstressed possessive forms usually come after the noun:

• το βιβλίο μου = my book
• η φίλη σου = your friend
• η ταυτότητά σου = your ID

So Greek structure here is different from English. You do not normally put σου before the noun in this kind of phrase.

Because adjectives in Greek agree with the noun they describe in gender, number, and case.

Ταυτότητα is:

• feminine
• singular
• nominative

So the adjective must also be feminine singular nominative:

• masculine: έγκυρος
• feminine: έγκυρη
• neuter: έγκυρο

That is why the sentence says η ταυτότητά σου δεν είναι πια έγκυρη.

Here πια means any longer, anymore, or no longer, depending on how you translate the whole sentence.

So:

• δεν είναι πια έγκυρη = it is no longer valid

It often appears with negation:

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

In Modern Greek, γιατί can mean both because and why. The meaning comes from context.

Here it introduces a reason:

• ...δεν είναι πια έγκυρη, γιατί έχει λήξει...
• ...is no longer valid, because it has expired...

If it were a question, the sentence structure and punctuation would show that:

• Γιατί έχει λήξει; = Why has it expired?

So in this sentence, γιατί clearly means because.

Έχει λήξει is the present perfect: has expired.

It is used here because the speaker is emphasizing a past event with a present result:

• it expired earlier
• and now it is still expired / no longer valid

Compare:

• έληξε = it expired / it did expire
  
  • simple past, more focused on the event itself
• έχει λήξει = it has expired
  
  • focuses on the current consequence

That makes έχει λήξει very natural in a sentence about a document no longer being valid.

The dictionary form is λήγω, which means to expire, to end, or to run out, depending on context.

Some useful forms are:

• λήγει = it expires / is expiring
• έληξε = it expired
• έχει λήξει = it has expired
• να λήξει = to expire / that it expire
• θα λήξει = it will expire

So έχει λήξει comes from the verb λήγω.

Greek often leaves the subject understood when it is already clear from context.

Here, after η ταυτότητά σου has already been mentioned, Greek does not need to repeat it:

• γιατί έχει λήξει = because it has expired

The understood subject is still η ταυτότητα. This kind of omission is very common in Greek.

Πρέπει is an impersonal verb-like form meaning it is necessary, one must, must, or need to, depending on context.

It is followed by να + a verb form:

• πρέπει να φύγω = I have to leave
• πρέπει να διαβάσεις = you have to study
• πρέπει να την ανανεώσεις = you must renew it

So πρέπει να is one of the main ways Greek expresses obligation or necessity.

Την is the direct object pronoun it, feminine singular, referring back to την ταυτότητα.

Because ταυτότητα is feminine singular:

• nominative: η
• accusative: την

So:

• πρέπει να την ανανεώσεις = you must renew it

Greek object pronouns usually come before the verb in structures like this:

• τον βλέπω = I see him
• την ξέρω = I know her/it
• θα την πάρω = I will take it
• να την ανανεώσεις = to renew it

Ανανεώσεις here is the aorist subjunctive form, used after να:

• να ανανεώσεις

This form is used because the sentence refers to a single complete action: renewing the ID once.

Compare:

• να την ανανεώσεις = to renew it / that you renew it once
• να την ανανεώνεις = to be renewing it regularly / repeatedly

Since an ID card is normally renewed as a one-time action, να την ανανεώσεις is the natural choice.

Yes. Πλέον can often replace πια in this kind of sentence:

• δεν είναι πλέον έγκυρη

Both mean no longer valid.

The difference is mostly one of style:

• πια is very common and everyday
• πλέον is a bit more formal or written

So the sentence with πια sounds perfectly natural in ordinary Greek.