Όταν τελειώσω το διάβασμα, θα βάλω τον χάρακα, τον μαρκαδόρο και το ψαλίδι πίσω στην κασετίνα, για να μην τα ψάχνω αύριο το πρωί.

In Greek, when όταν means when in a future-time sentence, it is normally followed by the subjunctive, not by θα.

So:

• Όταν τελειώσω = when I finish
• not όταν θα τελειώσω

The form τελειώσω is the aorist subjunctive of τελειώνω / τελειώνω κάτι. Here it refers to a single completed action: finishing the studying.

Compare:

• Όταν τελειώσω, θα βγω. = When I finish, I’ll go out.
• Όταν τελειώνω, είμαι κουρασμένος. = When I finish / whenever I finish, I am tired.
  This can sound more habitual or general depending on context.

So in your sentence, τελειώσω is used because the speaker means one future completion point: when I’m done studying.

Το διάβασμα is a noun meaning studying, schoolwork, or sometimes reading, depending on context.

It comes from διαβάζω, which can mean:

• read
• study

In this sentence, το διάβασμα most naturally means the studying / my studying / my schoolwork.

So:

• Όταν τελειώσω το διάβασμα = When I finish studying / When I finish my studying

The article το is very normal here. Greek often uses the article with abstract nouns or activities where English might not.

Yes. Βάλω is the aorist stem of βάζω.

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

Greek often uses:

• present/imperfective stem for ongoing, repeated, or habitual actions
• aorist/perfective stem for single completed actions

Here the speaker means one simple future action:

• I’ll put the ruler, marker, and scissors back

So θα βάλω is exactly what you would expect.

These are the direct objects of θα βάλω, so they appear in the accusative case, together with their definite articles.

• ο χάρακας → τον χάρακα
• ο μαρκαδόρος → τον μαρκαδόρο
• το ψαλίδι → το ψαλίδι

So the sentence is literally:

• I will put the ruler, the marker, and the scissors...

Greek usually keeps the article with each noun in a list like this, especially in careful or natural speech.

Greek treats ψαλίδι as a singular neuter noun, even though English uses the plural word scissors.

So:

• το ψαλίδι = the scissors
• ένα ψαλίδι = a pair of scissors

This is just a vocabulary difference between the two languages.

Here πίσω means back, not literally behind.

So:

• βάζω κάτι πίσω στην κασετίνα = put something back in the pencil case

This is very natural Greek.

Also:

• στην = σε + την
• κασετίνα = pencil case

So literally:

• back into the pencil case

In English we often say put it back in the pencil case, and Greek expresses that with πίσω στην κασετίνα.

Greek usually uses the article where English may not.

So:

• στην κασετίνα = in the pencil case

Using σε κασετίνα without the article would sound odd in this context unless you meant something more indefinite, like into a pencil case.

Since the speaker clearly has a specific pencil case in mind, στην κασετίνα is the natural choice.

Because στην is the full form of σε την.

In modern Greek, the final -ν in forms like στη(ν), τη(ν), τη(ς) may be kept or dropped depending on style, region, and pronunciation habits. Before consonants, both forms are often possible in many cases.

So you may see:

• στην κασετίνα
• στη κασετίνα

Both can be heard, but στην is very common in writing and perfectly correct.

This is an important aspect difference.

• ψάχνω here is the imperfective form
• ψάξω would be the perfective/aorist form

After για να (so that / in order to), Greek uses the subjunctive. The choice between imperfective and aorist depends on how the action is viewed.

για να μην τα ψάχνω

This suggests:

• so I won’t have to be looking for them
• so I’m not searching around for them

It gives a sense of an action with duration, effort, or possible repetition.

για να μην τα ψάξω

This would sound more like:

• so that I won’t search for them
• one more bounded, single search event

In this sentence, ψάχνω is very natural because the idea is:
I want to put them away now so I’m not looking all over for them tomorrow morning.

Because μη(ν) is the normal negation used with the subjunctive and in other non-indicative environments.

So:

• για να βρω = in order to find
• για να μην χάσω = so that I don’t lose
• για να μην τα ψάχνω = so that I won’t be looking for them

Greek does not use δεν here.
Δεν is used with the indicative, while μην is used with the subjunctive.

Because Greek very often uses neuter plural clitics like τα to refer back to a group of things/objects, even if the individual nouns have different grammatical genders.

The items are:

• τον χάρακα (masculine)
• τον μαρκαδόρο (masculine)
• το ψαλίδι (neuter)

But together they can naturally be referred to as τα = them.

This is very common when talking about inanimate objects as a group.

Because Greek often uses a bare time expression for parts of the day.

So:

• αύριο το πρωί = tomorrow morning
• αύριο το βράδυ = tomorrow evening
• σήμερα το απόγευμα = this afternoon

No preposition is needed.

Yes, in a sense.

Greek often leaves out possessives like μου (my) when ownership is obvious from context.

So:

• πίσω στην κασετίνα naturally means back in my/the pencil case, depending on context.

If the speaker wanted to be explicit, they could say:

• πίσω στην κασετίνα μου

But it is not necessary here.

Because Όταν τελειώσω το διάβασμα is a subordinate time clause at the beginning of the sentence.

Greek punctuation often puts a comma after an introductory clause like this, just as English usually does:

• When I finish studying, I’ll put...

So the comma marks the end of the when-clause and the start of the main clause:

• Όταν τελειώσω το διάβασμα,
• θα βάλω...

Literally, yes, but that would usually sound unnatural in English.

The Greek noun διάβασμα can cover both:

• reading
• studying
• schoolwork

In this context, the natural English meaning is more likely:

• When I finish studying
• When I finish my schoolwork

So the literal form can help you understand the Greek, but it is not always the best English translation.