Ο προϊστάμενος οφείλει να απαντήσει σήμερα, γιατί η προθεσμία λήγει αύριο.

Ο προϊστάμενος means the manager, the supervisor, or the person in charge.

• ο = the masculine singular definite article
• προϊστάμενος = a masculine singular noun/adjectival form meaning supervisor / manager

In this sentence, ο προϊστάμενος is the subject of the verb οφείλει.

The two dots on ϊ show that the vowels are pronounced separately.

So προϊ- is pronounced roughly as pro-i-, not as a single sound. Without the diaeresis, οι is normally pronounced as i in Modern Greek. The diaeresis tells you not to combine them that way here.

So the beginning is:

• προϊ- = pro-i not
• προι- = pri

Here οφείλει means must, is obliged to, or is required to.

The basic verb is οφείλω. It can mean:

• to owe money or something
• to be obliged to / should / must

In this sentence, it clearly has the second meaning: Ο προϊστάμενος οφείλει να απαντήσει σήμερα
= The manager must answer today

This is a bit more formal than simply using πρέπει.

Modern Greek does not normally use the infinitive the way English does. Instead, it usually uses να + finite verb form.

So where English says:

• must answer
• has to answer
• to answer

Greek often says:

• να απαντήσει

This structure is extremely common in Modern Greek and is one of the first big differences English speakers notice.

Because να απαντήσει presents the action as one complete act: giving an answer.

• να απαντήσει = to answer once / to give the answer
• να απαντά = to be answering / to answer habitually or repeatedly

In this sentence, the idea is a single required action that needs to happen today, so να απαντήσει is the natural choice.

It is not past here. απαντήσει is the perfective subjunctive form used after να.

So:

• να απαντήσει = to answer / that he answer
• not he answered

English speakers often confuse this form with a past tense because it resembles forms used in other contexts, but here it simply expresses the required action after να.

Because both are time adverbs.

• σήμερα = today
• αύριο = tomorrow

Greek, like English, often uses such words directly:

• today
• tomorrow

So:

• να απαντήσει σήμερα = to answer today
• λήγει αύριο = expires tomorrow

No preposition is needed.

Here γιατί means because.

So: γιατί η προθεσμία λήγει αύριο
= because the deadline expires tomorrow

The same word γιατί can also mean why in a question:

• Γιατί άργησες; = Why were you late?

Context tells you which meaning is intended. In your sentence, it introduces a reason, so it means because.

Η προθεσμία means the deadline.

It is feminine because προθεσμία is a feminine noun in Greek, so it takes the feminine article η.

Here it is in the nominative singular because it is the subject of λήγει:

• η προθεσμία λήγει = the deadline expires

You simply have to learn the grammatical gender of nouns as part of the word.

Greek often uses the present tense for scheduled or certain future events, just as English sometimes does.

Compare English:

• The train leaves tomorrow
• The deadline expires tomorrow

Greek does the same:

• η προθεσμία λήγει αύριο

So although the form is present, the meaning is future because αύριο makes the time clear.

Not completely. Greek word order is more flexible than English word order.

The neutral order here is: Ο προϊστάμενος οφείλει να απαντήσει σήμερα, γιατί η προθεσμία λήγει αύριο.

But Greek could move parts around for emphasis, for example:

• Σήμερα ο προϊστάμενος οφείλει να απαντήσει...
• Η προθεσμία λήγει αύριο, γι' αυτό ο προϊστάμενος οφείλει να απαντήσει σήμερα.

Even so, the original version sounds natural and straightforward.

The comma separates the main clause from the clause giving the reason.

Main clause:

• Ο προϊστάμενος οφείλει να απαντήσει σήμερα

Reason clause:

• γιατί η προθεσμία λήγει αύριο

So the comma helps show the structure: The manager must answer today, because the deadline expires tomorrow.

In Greek punctuation, this is very normal.

A rough pronunciation guide is:

O pro-i-STA-me-nos o-FI-li na a-pan-TI-si SI-me-ra, ya-TI i pro-the-SMI-a LI-yi AV-ri-o.

A few useful points:

• προϊ- = pro-i
• ει in οφείλει sounds like i
• γιατί sounds roughly like ya-TI
• λήγει is roughly LI-yi
• αύριο is roughly AV-ri-o

This is only an approximation, but it helps an English speaker get started.