Θα βάλω την απόδειξη μέσα σε ένα ντοσιέ, για να μη χαθεί.

In Modern Greek, θα is the future particle.

So:

• βάζω = I put / I am putting
• θα βάλω = I will put

The form βάλω here is the perfective form, used for a single complete action. It fits this sentence because putting the receipt into a folder is seen as one completed act.

If you said θα βάζω, that would usually suggest something more repeated, habitual, or ongoing, like I will be putting or I will put regularly.

Because την απόδειξη is the direct object of the verb βάλω.

Greek changes articles and often noun endings depending on grammatical case.

Here:

• η απόδειξη = the receipt as the subject (nominative)
• την απόδειξη = the receipt as the object (accusative)

So in this sentence, the speaker is doing something to the receipt, which is why Greek uses the accusative form:

• Θα βάλω την απόδειξη... = I will put the receipt...

Yes. Απόδειξη can mean:

• receipt
• proof

In this sentence, it clearly means receipt, because the speaker is putting it into a folder so it does not get lost.

This is a common feature in Greek: one word may cover meanings that English separates into different words.

Μέσα σε means inside, into, or in with a stronger sense of being within something.

So:

• σε ένα ντοσιέ = in a folder
• μέσα σε ένα ντοσιέ = inside a folder

Using μέσα σε makes the image more explicit. It emphasizes that the receipt will be placed inside the folder, not just associated with it in some looser way.

In many situations, σε alone would also be possible, but μέσα σε sounds more vivid and precise here.

Ντοσιέ is a loanword, and it usually means something like:

• folder
• file
• sometimes binder, depending on context

In this sentence, folder is probably the most natural translation.

It is very common in everyday Greek, even though it is not originally an ancient Greek-style word.

Because ντοσιέ is treated as neuter singular, so the correct form of the indefinite article is ένα.

Compare:

• ένας for masculine nouns
• μια / μία for feminine nouns
• ένα for neuter nouns

So:

• ένα ντοσιέ = a folder

Για να means so that or in order to.

It introduces a purpose clause: it tells you why the speaker will put the receipt in the folder.

So the structure is:

• Θα βάλω την απόδειξη μέσα σε ένα ντοσιέ = I will put the receipt in a folder
• για να μη χαθεί = so that it won’t get lost

Together, the second part explains the purpose of the first part.

Because μη is used to negate subjunctive-type structures, and δεν is used to negate indicative statements.

After να, Greek normally uses μη(ν), not δεν.

So:

• δεν χάνεται = it is not getting lost / it doesn’t get lost
• να μη χαθεί = so that it does not get lost / so that it won’t get lost

That is why για να μη χαθεί is correct.

You may also see μην instead of μη. The choice between μη and μην often depends on sound and style. Before a consonant, μη is very common.

Χαθεί is the aorist subjunctive passive/mediopassive form, third person singular, from the verb χάνομαι / related to χάνω.

In simple learner-friendly terms, here it means:

• get lost
• be lost

So:

• να μη χαθεί = so that it doesn’t get lost / so that it won’t get lost

The aorist form is used because the idea is one possible event: the receipt getting lost.

Because χαθεί fits the idea of a single event better.

Compare the feeling:

• να μη χαθεί = so that it doesn’t get lost as a one-time event
• να μη χάνεται = more like so that it isn’t getting lost / doesn’t keep getting lost, which sounds less natural here

A receipt is not something that repeatedly gets lost in this context. The speaker wants to prevent one possible loss, so χαθεί is the natural choice.

The subject is understood to be the receipt.

Greek often leaves out pronouns and even repeated nouns when the meaning is clear from context.

So in:

• Θα βάλω την απόδειξη... για να μη χαθεί

the understood meaning is:

• ...so that the receipt doesn’t get lost

Greek does not need to repeat η απόδειξη here.

The comma separates the main clause from the purpose clause:

• Θα βάλω την απόδειξη μέσα σε ένα ντοσιέ,
• για να μη χαθεί.

In Greek, punctuation with clauses like this is often fairly natural and readable rather than something learners need to overthink at first. The comma helps show that the second part explains the purpose of the first.

You may sometimes see similar sentences without a comma, especially in less formal writing, but the comma here is perfectly normal.

Yes. Greek offers a few natural alternatives, depending on nuance.

For example:

• Θα βάλω την απόδειξη σε ένα ντοσιέ, για να μη χαθεί.
  This is slightly less explicit than μέσα σε, but still natural.

• Θα βάλω την απόδειξη σε ένα ντοσιέ για να μην τη χάσω.
  This means I’ll put the receipt in a folder so I don’t lose it.

That last version shifts the focus a little:

• να μη χαθεί = so that it doesn’t get lost
• να μην τη χάσω = so that I don’t lose it

Both are natural, but they are not exactly identical in perspective.