Οφείλω να απαντήσω σήμερα, γιατί η προθεσμία λήγει αύριο.

Here οφείλω να... means I must / I ought to...

It is similar to πρέπει να..., but οφείλω is often a bit more formal and can sound more like a duty, obligation, or responsibility. It can also literally mean I owe in other contexts, for example Οφείλω χρήματα = I owe money.

So in this sentence, Οφείλω να απαντήσω σήμερα suggests a fairly strong sense of obligation.

In Modern Greek, you usually do not use an infinitive the way English does with to answer.

Instead, Greek normally uses να + verb.

So:

• να απαντήσω = roughly to answer / that I answer

After verbs like οφείλω, πρέπει, θέλω, μπορώ, Greek uses this να structure:

• Θέλω να φύγω = I want to leave
• Πρέπει να διαβάσω = I must study

So Οφείλω να απαντήσω is the normal Greek way to say I must answer.

Because after να, Greek often uses a subjunctive form, and here απαντήσω is the aorist subjunctive.

The basic idea is:

• να απαντήσω focuses on one complete action: to give an answer
• να απαντώ would suggest something more ongoing, repeated, or habitual

In this sentence, the speaker is talking about one specific act of answering, so απαντήσω is the natural choice.

Not exactly by itself.

Απαντήσω here is not a future tense form on its own. It is the form used after να. The future meaning comes from the context, especially σήμερα.

So:

• να απαντήσω σήμερα = to answer today
• θα απαντήσω would be the straightforward future: I will answer

Greek often lets time words like σήμερα, αύριο, σε λίγο show when something happens.

Greek often leaves out subject pronouns when they are not needed, because the verb ending already shows the person.

Here:

• οφείλω = I must / I ought
• απαντήσω = that I answer

So there is no need to say εγώ unless you want emphasis:

• Εγώ οφείλω να απαντήσω σήμερα = I am the one who must answer today

This is very normal in Greek.

Η is the feminine singular definite article, meaning the.

Greek uses the definite article very often, and here η προθεσμία means the deadline.

The noun προθεσμία is:

• feminine
• singular
• in the nominative here, because it is the subject of λήγει

So:

• η προθεσμία = the deadline

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

• The train leaves tomorrow
• Η προθεσμία λήγει αύριο

So λήγει αύριο literally looks like expires tomorrow / is expiring tomorrow, but it naturally means expires tomorrow.

You could also hear:

• η προθεσμία θα λήξει αύριο

That is also correct, but λήγει αύριο is very natural for a scheduled deadline.

Yes. Γιατί can mean both:

• because
• why

In this sentence, it means because:

• ..., γιατί η προθεσμία λήγει αύριο.
• ..., because the deadline expires tomorrow.

When it means why, the context or punctuation makes that clear:

• Γιατί φεύγεις; = Why are you leaving?

So the word is the same, but the sentence tells you which meaning it has.

Yes, Greek word order is more flexible than English word order.

The sentence as given is natural:

• Οφείλω να απαντήσω σήμερα, γιατί η προθεσμία λήγει αύριο.

But you could also move things around for emphasis, for example:

• Σήμερα οφείλω να απαντήσω, γιατί η προθεσμία λήγει αύριο.
• Οφείλω σήμερα να απαντήσω, γιατί η προθεσμία λήγει αύριο.

These all mean roughly the same thing, but the emphasis changes slightly. Greek often uses word order to highlight what feels most important.

Λήγει comes from λήγω and means ends, expires, or comes to an end, depending on context.

With προθεσμία it usually means:

• the deadline expires
• the deadline ends

So η προθεσμία λήγει αύριο is a very natural Greek way to say:

• the deadline expires tomorrow

The accent marks show which syllable is stressed.

A rough pronunciation guide is:

• Οφείλω → o-FEE-lo
• να → na
• απαντήσω → a-pan-DEE-so
  
  • the ντ here is often pronounced like d
• σήμερα → SEE-me-ra
• γιατί → ya-TEE
• η → ee
• προθεσμία → pro-thes-MEE-a
• λήγει → LEE-yi
• αύριο → AV-rio

So the whole sentence is roughly:

o-FEE-lo na a-pan-DEE-so SEE-me-ra, ya-TEE ee pro-thes-MEE-a LEE-yi AV-rio

This is only an approximation, but it is useful for seeing where the stress goes.