Αν ο πονοκέφαλος δεν φύγει, θα πάρω άλλο ένα παυσίπονο το βράδυ.

Because after αν when Greek talks about a possible future situation, it normally uses the subjunctive, not the ordinary present indicative.

So:

• αν ... δεν φύγει = if ... doesn’t go away
• φύγει here is the aorist subjunctive of φεύγω

This is very common in Greek:

• Αν βρέξει, θα μείνουμε μέσα. = If it rains, we’ll stay inside.
• Αν έρθει, θα μιλήσουμε. = If he comes, we’ll talk.

Using δεν φεύγει would sound more like a statement about what is happening generally or right now, not the usual future if pattern.

Yes, φεύγω often means leave, but it can also mean go away, especially with symptoms, pain, or problems.

So:

• ο πονοκέφαλος δεν φεύγει / δεν φύγει = the headache doesn’t go away

This is a very natural Greek way to talk about symptoms disappearing.

Similar examples:

• Ο πόνος δεν φεύγει. = The pain isn’t going away.
• Η κούραση δεν έφυγε ακόμα. = The tiredness hasn’t gone away yet.

Greek often uses the definite article where English would not.

So ο πονοκέφαλος literally looks like the headache, but in natural English the sentence is translated simply as if the headache doesn’t go away or even if I still have a headache depending on context.

The article is very normal here. Greek uses articles much more regularly than English.

Other common examples:

• Έχω τον πυρετό. = literally I have the fever
• Με πονάει το κεφάλι. = literally My head hurts / the head hurts me

Yes. In Modern Greek, the future is usually formed with θα + verb.

Here:

• θα πάρω = I will take

The verb πάρω is the form used after θα. It comes from παίρνω (to take).

A few examples:

• Θα πάρω ένα χάπι. = I’ll take a pill.
• Θα φύγω αύριο. = I’ll leave tomorrow.
• Θα δω την ταινία. = I’ll watch the movie.

So in your sentence, θα πάρω is simply I’ll take.

Because after θα, Greek usually uses a different form from the plain present-tense form.

Compare:

• παίρνω = I take / I am taking
• θα πάρω = I will take

This is a very common pattern in Greek verbs, especially with verbs that have different stems:

• βλέπω → θα δω
• έρχομαι → θα έρθω
• παίρνω → θα πάρω

So πάρω is the correct form after θα.

Άλλο ένα means another one / one more.

In this sentence:

• θα πάρω άλλο ένα παυσίπονο = I’ll take another painkiller / one more painkiller

This is the most natural order in Greek for this meaning.

Compare:

• άλλο ένα = one more / another
• ένα άλλο can also exist, but it more strongly means a different one

So there is a slight difference:

• Θέλω άλλο ένα καφέ. = I want one more coffee.
• Θέλω ένα άλλο καφέ. = I want a different coffee.

In your sentence, the idea is clearly one more painkiller, so άλλο ένα is right.

It is in the accusative, because it is the direct object of θα πάρω (I will take).

• παίρνω κάτι = I take something
• so παυσίπονο is the thing being taken

For neuter nouns like παυσίπονο, the nominative and accusative singular are the same in form:

• το παυσίπονο = nominative or accusative singular, depending on function

Here:

• άλλο ένα παυσίπονο = another painkiller
  This whole phrase is the object of the verb.

Both can occur in Greek, but το βράδυ is a very common way to say in the evening / tonight / at night, depending on context.

The article is often used in time expressions:

• το πρωί = in the morning
• το μεσημέρι = at noon / in the afternoon
• το βράδυ = in the evening / at night

So:

• θα πάρω άλλο ένα παυσίπονο το βράδυ = I’ll take another painkiller tonight / in the evening

The exact English translation depends on context.

Yes, Greek word order is flexible.

Your sentence says:

• Αν ο πονοκέφαλος δεν φύγει, θα πάρω άλλο ένα παυσίπονο το βράδυ.

That is completely natural. But Greek could also say:

• Αν ο πονοκέφαλος δεν φύγει, το βράδυ θα πάρω άλλο ένα παυσίπονο.

The difference is mostly about emphasis.

• Putting το βράδυ later sounds neutral.
• Putting το βράδυ earlier can emphasize the time.

Greek often moves pieces around more freely than English, as long as the grammar is clear.

Yes. This is a standard future conditional pattern in Greek:

• Αν + subjunctive, θα + verb

So the structure is:

• Αν ο πονοκέφαλος δεν φύγει = If the headache doesn’t go away
• θα πάρω άλλο ένα παυσίπονο = I’ll take another painkiller

This is one of the most useful Greek patterns to learn.

More examples:

• Αν κουραστώ, θα ξεκουραστώ λίγο. = If I get tired, I’ll rest a bit.
• Αν δεν κοιμηθώ καλά, θα είμαι χάλια αύριο. = If I don’t sleep well, I’ll feel awful tomorrow.

That is a very good question, because learners often expect μη(ν) with the subjunctive.

In Modern Greek:

• δεν is used in clauses with αν
• μη(ν) is used in some other subjunctive environments, such as commands, wishes, fears, or after certain particles

So:

• Αν δεν φύγει... = correct
• Αν μην φύγει... = not correct here

Compare:

• Αν δεν έρθει, θα φύγουμε. = If he doesn’t come, we’ll leave.
• Να μην αργήσεις. = Don’t be late.

So even though φύγει is subjunctive in form, the negative in this if-clause is δεν.

Yes, in many contexts Greek speakers could also say:

• Αν ο πονοκέφαλος δεν περάσει...

This also means If the headache doesn’t go away / doesn’t pass.

The verb περνάω / περάσω is also very common with illnesses, pain, and unpleasant states:

• Μου πέρασε ο πονοκέφαλος. = My headache went away.
• Δεν μου πέρασε ακόμα. = It hasn’t gone away yet.

So both δεν φύγει and δεν περάσει are natural, though individual speakers may prefer one over the other.

A very natural Greek way is:

• Αν ο πονοκέφαλος δεν φύγει, θα πάρω άλλο ένα παυσίπονο το βράδυ.

This already expresses that idea well: if the headache doesn’t go away...

If you want something closer to unless, Greek often prefers to rephrase it rather than use a direct one-word equivalent in everyday speech.

For example:

• Εκτός αν μου περάσει ο πονοκέφαλος, θα πάρω άλλο ένα παυσίπονο το βράδυ.

But the original sentence is simpler and more common.

Yes, exactly.

πονοκέφαλος is a compound word:

• πόνος = pain
• κεφάλι = head

So historically it is something like head-pain, which is why it means headache.

Greek has many useful compound words like this, and noticing their parts can help vocabulary learning.

For example:

• οδοντόκρεμα = toothpaste
• ποδόσφαιρο = football / soccer
• ασθενοφόρο = ambulance

So πονοκέφαλος is a nice example of how Greek builds words.

A helpful rough pronunciation is:

An o ponoKÉfalos den FÍyi, tha PÁro Álo Éna pafSÍpono to VRÁdi.

A few notes:

• Αν = an
• πονοκέφαλος has stress on κέ
• φύγει sounds roughly like FEE-yee, with the stress on the first syllable
• θα πάρω = tha PA-ro
• παυσίπονο has stress on σί
• βράδυ has stress on βρά

This is only an approximation, but it can help you hear the rhythm of the sentence.