Πρέπει να έχω αφήσει την οδοντόβουρτσά μου στο μπάνιο, γιατί δεν τη βρίσκω πουθενά.

Here πρέπει expresses deduction, not obligation.

So Πρέπει να έχω αφήσει... means I must have left... in the sense of that’s probably what happened.

The speaker is reasoning from evidence:

• I can’t find it anywhere
• so I must have left it in the bathroom

If it were obligation, the sentence would be about something the speaker is required to do, not something they are concluding about the past.

Because in Modern Greek, πρέπει is normally followed by a να-clause.

Greek does not use an English-style infinitive after words like must, want, can, etc. Instead, it usually uses:

• πρέπει να + verb

So:

• πρέπει να έχω αφήσει = I must have left

This is a very common Greek pattern.

Έχω αφήσει is the perfect form of αφήνω.

It means:

• I have left
• I have left behind

In this sentence, together with πρέπει να, it gives the idea:

• I must have left

So the whole structure is basically:

• πρέπει να έχω αφήσει = I must have left

This is a natural way to talk about a past action that the speaker is inferring now.

Because έχω αφήσει helps express the idea of a completed past action with present relevance.

The speaker is talking about something that probably happened earlier, but the conclusion matters now, because they cannot find the toothbrush now.

So:

• άφησα = I left / I did leave
• έχω αφήσει = I have left
• πρέπει να έχω αφήσει = I must have left

In this kind of sentence, the perfect is a very natural match for English must have + past participle.

In Greek, possessed nouns usually take the definite article.

So Greek commonly says:

• η οδοντόβουρτσά μου
  literally: the toothbrush my

but it means:

• my toothbrush

That is completely normal Greek structure.

So:

• την οδοντόβουρτσά μου = my toothbrush in the accusative

Because the short possessive forms like μου, σου, του, της usually come after the noun.

So Greek normally says:

• η οδοντόβουρτσά μου = my toothbrush
• το βιβλίο μου = my book
• η τσάντα σου = your bag

This is the standard, neutral pattern.

If you want emphasis, Greek can use a different structure, for example:

• η δική μου οδοντόβουρτσα = my toothbrush / the toothbrush that is mine

But in ordinary speech, noun + μου is the usual choice.

This is an accent rule.

When a word like οδοντόβουρτσα is followed by an enclitic word such as μου, Greek spelling changes the accent placement:

• οδοντόβουρτσα
• οδοντόβουρτσά μου

This happens very often with possessive clitics like μου, σου, του, etc.

So the changed accent is not a different word or case ending by itself; it is mainly a matter of correct Greek accentuation in this environment.

Because στο is the contraction of:

• σε + το = στο

So:

• στο μπάνιο = in the bathroom / to the bathroom, depending on context

Here it means in the bathroom.

Also, μπάνιο can mean bath or bathroom depending on context. In this sentence, bathroom is the natural meaning.

Τη is the feminine singular direct object pronoun, referring back to την οδοντόβουρτσά μου.

So:

• τη = it here, specifically the toothbrush

The full form is την, but in many contexts Greek drops the final -ν before certain consonants. Since the next word is βρίσκω, it is very common to write:

• τη βρίσκω

So:

• δεν τη βρίσκω = I can’t find it

Because the speaker is describing a current situation:

• I’m not finding it now
• I can’t find it now

So Greek uses the present:

• δεν τη βρίσκω

This is very natural Greek for a present inability or ongoing unsuccessful search.

English often uses can’t find, while Greek simply uses the present verb with negation.

Πουθενά means anywhere / nowhere, depending on the sentence.

In a negative sentence like this:

• δεν τη βρίσκω πουθενά

it means:

• I can’t find it anywhere

Greek often uses words like πουθενά, κανείς, τίποτα together with δεν in negative sentences.

So literally, the structure is something like:

• not find it anywhere

which sounds perfectly natural in Greek.

Here it means because.

Greek γιατί can mean both:

• why?
• because

You tell which meaning it has from the context and punctuation.

In this sentence, it connects two statements:

• I must have left my toothbrush in the bathroom, because I can’t find it anywhere

So here it is clearly because, not why.