Πρέπει να ξέχασα το ντοσιέ στο γραφείο, γιατί δεν το βρίσκω πουθενά στο σπίτι.

Because πρέπει να has more than one use in Greek.

• obligation: Πρέπει να φύγω = I have to leave
• logical conclusion / deduction: Πρέπει να ξέχασα = I must have forgotten

In this sentence, the second part gives the reason for the conclusion:

γιατί δεν το βρίσκω πουθενά στο σπίτι
= because I can’t find it anywhere in the house

So the speaker is not talking about duty. They are making a guess based on evidence.

Yes, πρέπει να ξέχασα is natural in everyday Greek and means I must have forgotten.

A more explicitly periphrastic version is:

πρέπει να έχω ξεχάσει

Both are used for deduction about the past. Very roughly:

• πρέπει να ξέχασα = more direct, conversational
• πρέπει να έχω ξεχάσει = also very common, slightly more analytical in form

So a learner should understand both.

Because forgetting the folder is seen as a single completed event.

The aorist in Greek often presents an action as a whole:

• ξέχασα = I forgot
• ξεχνούσα would suggest an ongoing or repeated past situation, which does not fit here

The speaker is thinking of one moment in the past when they probably left the folder behind.

Το ντοσιέ means something like folder, file, or binder, depending on context.

It is a borrowed word, and in Modern Greek it is usually treated as neuter, which is why you get:

• το ντοσιέ
• δεν το βρίσκω = I can’t find it

So the pronoun το agrees with ντοσιέ.

That το is the direct-object pronoun it.

It refers back to το ντοσιέ:

• το ντοσιέ = the folder
• δεν το βρίσκω = I can’t find it

This is completely normal Greek. English also does the same thing across clauses:

I must have forgotten the folder at the office, because I can’t find it anywhere at home.

Greek normally uses negative concord. That means words like πουθενά, ποτέ, κανείς, and τίποτα usually appear together with δεν or μην.

So:

• δεν το βρίσκω πουθενά = I can’t find it anywhere
• literally, it looks like I don’t find it nowhere, but that is just how Greek grammar works

So for a learner, the important point is: with these negative-type words, Greek usually still keeps δεν.

Το γραφείο can mean either:

• office
• desk

Context decides.

In this sentence, at the office is probably the more likely meaning, because it is contrasted with στο σπίτι. The speaker seems to mean:

I must have forgotten the folder at the office, because I can’t find it anywhere at home.

But in another context, στο γραφείο could also mean on the desk or at the desk.

Because the speaker is describing their current situation now.

• δεν το βρίσκω = I’m not finding it / I can’t find it
• it shows an ongoing present attempt or present result of searching

Greek often uses the simple present where English naturally says can’t find.

So the idea is: Right now I’m searching at home, and I still can’t find it.

Στο is the contraction of σε + το.

So:

• στο σπίτι = in the house / at home

Greek very often uses the article in place expressions. Here στο σπίτι is the normal form.

You can also hear bare σπίτι in some patterns, especially with motion verbs:

• πάω σπίτι = I’m going home

But in this sentence, with the meaning anywhere in the house / at home, στο σπίτι is the expected choice.

Here γιατί means because.

It introduces the reason for the speaker’s conclusion:

Πρέπει να ξέχασα το ντοσιέ στο γραφείο, γιατί δεν το βρίσκω πουθενά στο σπίτι.
= I must have forgotten the folder at the office, because I can’t find it anywhere at home.

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

• Γιατί άργησες; = Why were you late?

So context tells you whether it means because or why.

In Modern Greek, unstressed object pronouns usually come before a finite verb.

So:

• το βρίσκω = I find it
• τον βλέπω = I see him
• την ξέρω = I know her

That is why the sentence says:

δεν το βρίσκω

This word order is normal and expected.

Yes. That would also be natural.

• δεν το βρίσκω = I can’t find it / I’m not finding it
• δεν μπορώ να το βρω = I can’t find it in a more explicit am not able to sense

The original version is very natural and a bit more direct for everyday speech. It focuses on the practical result: the speaker has looked around, and the folder is nowhere to be found.