Μέχρι τις δώδεκα θα έχω τελειώσει τη δουλειά.

In this sentence, Μέχρι τις δώδεκα is best understood as by twelve o’clock.

Why? Because the main verb is a completed action: θα έχω τελειώσει = I will have finished. So the idea is that the work will be completed no later than twelve.

• Μέχρι can mean until in some contexts.
• But with a completed event like this, it often works like by.

So:

• Μέχρι τις δώδεκα θα έχω τελειώσει τη δουλειά.
  = By twelve, I will have finished the work.

Greek often uses the definite article with clock times:

• στις δώδεκα = at twelve
• μέχρι τις δώδεκα = by/until twelve

So τις is not unusual here; it is the normal way to refer to a specific hour.

Grammatically, τις δώδεκα is in the accusative, because the preposition μέχρι takes that form here.

You may sometimes hear μέχρι δώδεκα in casual speech, but μέχρι τις δώδεκα is very standard and natural.

Because θα έχω τελειώσει is the future perfect, and it emphasizes that the action will be already completed by a certain time.

Compare:

• Θα τελειώσω τη δουλειά.
  = I will finish the work.
  This just says the action will happen in the future.

• Μέχρι τις δώδεκα θα έχω τελειώσει τη δουλειά.
  = By twelve I will have finished the work.
  This stresses completion before a deadline / reference point.

So the future perfect is used because the sentence is about the state of completion by twelve.

It is formed like this:

• θα = future marker
• έχω = I have
• τελειώσει = perfective form used with έχω to make perfect tenses

So literally the structure is something like:

• θα + έχω + τελειώσει

This gives the meaning I will have finished.

A useful comparison:

• έχω τελειώσει = I have finished
• θα έχω τελειώσει = I will have finished

Τελειώσει is the form used after έχω to build the perfect tenses. It comes from the perfective stem of τελειώνω.

For learners, the easiest practical way to think of it is:

• τελειώνω = I finish / am finishing
• τελείωσα = I finished
• έχω τελειώσει = I have finished
• θα έχω τελειώσει = I will have finished

It is not a separate fully conjugated tense by itself here; it is part of the compound verb phrase with έχω.

Because τη δουλειά is the direct object of the verb.

• η δουλειά = nominative, used for the subject
• τη δουλειά = accusative, used for the object

Here, the thing being finished is the work, so Greek uses the accusative:

• θα έχω τελειώσει τη δουλειά

Also, τη is the common spoken/written form of την before a consonant:

• την δουλειά and τη δουλειά are both possible
• τη δουλειά is very common in modern Greek

Yes. Greek word order is more flexible than English, and that version is also natural:

• Μέχρι τις δώδεκα θα έχω τελειώσει τη δουλειά.
• Θα έχω τελειώσει τη δουλειά μέχρι τις δώδεκα.

Both mean the same thing.

The version with Μέχρι τις δώδεκα at the beginning gives a bit more emphasis to the time limit: By twelve...

So the difference is mostly one of focus/emphasis, not basic meaning.

Yes, it is natural and correct.

That said, in everyday conversation Greek often uses the simple future where English might strongly prefer future perfect, especially if the context already makes the deadline clear.

So a Greek speaker might also say:

• Μέχρι τις δώδεκα θα τελειώσω τη δουλειά.

This can also mean I’ll finish the work by twelve.

But θα έχω τελειώσει is more explicit about completion before that time, so it is a very good choice here.

Yes, in other kinds of sentences μέχρι can clearly mean until. For example:

• Θα δουλεύω μέχρι τις δώδεκα.
  = I will be working until twelve.

That describes an action continuing up to twelve.

But in your sentence:

• θα έχω τελειώσει = completed by then

So the meaning becomes by twelve, not I will keep finishing until twelve.

In short:

• continuous action
  • μέχρι → often until
• completed action
  • μέχρι → often by

No. Να is not used here.

You use να in structures like:

• Θέλω να τελειώσω τη δουλειά.
  = I want to finish the work.

But the future perfect is formed with:

• θα + έχω + τελειώσει

So να does not belong in this sentence.

Because Greek usually does not need subject pronouns unless there is emphasis or contrast.

The verb έχω already shows the person:

• έχω = I have

So θα έχω τελειώσει already means I will have finished.

You could say:

• Εγώ θα έχω τελειώσει τη δουλειά μέχρι τις δώδεκα.

But that would add emphasis, something like:

• I will have finished the work by twelve.

Without εγώ, the sentence is more neutral and natural in most situations.