Learn Hardcore Greek: Όταν τελειώσει η εκδρομή, το δάσος θα είναι πιο καθαρό και με λιγότερα σκουπίδια. - When the excursion is over, the forest will be cleaner and with less trash.

In Greek, when you talk about a future event introduced by Όταν (when), you normally use the subjunctive mood (without θα) in the όταν-clause, and the future (θα + present) in the main clause.

Όταν τελειώσει η εκδρομή = When the trip finishes / has finished (future time, but subjunctive)
το δάσος θα είναι… = the forest will be… (future tense)

τελειώσει is the aorist subjunctive, 3rd person singular, of the verb τελειώνω / τελειώσω.

So the pattern is:

Όταν + subjunctive, θα + verb
Όταν τελειώσει, θα είναι…

You would not normally say Όταν θα τελειώσει η εκδρομή in standard Greek.

So is τελειώσει a past tense because it’s “aorist”?

No. Here τελειώσει is aorist subjunctive, and in this context it refers to the future, not the past.

Aorist subjunctive in Greek does not mean “past”; it means a single, complete event (aspect), and the actual time (past/present/future) is given by context or other words.
With Όταν referring to a future time, the aorist subjunctive means “when it has finished / when it finishes” in the future.

So:

Όταν τελειώσει η εκδρομή… = When the trip is over… (future event, shown as a completed whole).

Could I also say Όταν η εκδρομή τελειώσει instead of Όταν τελειώσει η εκδρομή?

Yes, Όταν η εκδρομή τελειώσει is grammatically correct and means the same thing.

Differences:

Όταν τελειώσει η εκδρομή is the more natural, more common order for this kind of sentence.
Όταν η εκδρομή τελειώσει is possible but sounds a bit more marked / emphatic on η εκδρομή, or simply more formal or literary in many contexts.

In everyday speech, the original order (Όταν τελειώσει η εκδρομή) is what you will usually hear.

Why does the sentence say η εκδρομή with the article η? Could I drop it and just say Όταν τελειώσει εκδρομή?

You cannot drop the article here. Όταν τελειώσει εκδρομή is ungrammatical in standard Greek.

In Greek, specific countable nouns almost always take a definite article, even where English might allow no article:

η εκδρομή = the excursion / the trip (a particular trip that both speakers know about)
Greek likes definite articles much more than English; they’re used for known, specific things, often where English might say “when the trip is over” or even “when the trip’s over” without emphasizing that it’s “the one we’ve planned”.

So η here is just the normal definite article: η εκδρομή = “the excursion/trip”.

Why is it καθαρό and not καθαρός or καθαρή?

The adjective must agree with the noun δάσος in gender, number, and case.

το δάσος = neuter, singular, nominative
So the adjective must also be neuter, singular, nominative:
- καθαρό (neuter)
- καθαρός (masculine)
- καθαρή (feminine)

Because δάσος is neuter, the correct form is:

το δάσος θα είναι πιο καθαρό
the forest will be cleaner

What is the difference between πιο καθαρό and καθαρότερο? Are both correct?

Both are correct and both mean “cleaner”.

πιο καθαρό = more clean / cleaner (analytic comparative)
καθαρότερο = cleaner (synthetic comparative)

In modern everyday Greek:

The πιο + adjective form (πιο καθαρό) is more common and neutral.
The -ότερος form (καθαρότερο) is also correct, but it can sound:
- more formal,
- more “careful” or literary,
- or simply less common in casual speech for many adjectives.

So in this sentence, πιο καθαρό is the most natural choice.

Why is it λιγότερα σκουπίδια and not λιγότερο σκουπίδια?

Λιγότερα is an adjective here and must agree with σκουπίδια:

σκουπίδια = neuter, plural (literally “garbage items / pieces of trash”)
So the comparative of λίγα (few) in neuter plural is λιγότερα (fewer).

Forms:

λίγο – λιγότερο (neuter singular, or adverb “less”)
λίγα – λιγότερα (neuter plural, “few / fewer”)

Because σκουπίδια is neuter plural, you say:

λιγότερα σκουπίδια = fewer pieces of trash / less trash

Λιγότερο σκουπίδια is wrong because λιγότερο wouldn’t agree in number with the plural noun.

Why does the sentence say με λιγότερα σκουπίδια instead of just λιγότερα σκουπίδια or something with έχει?

The phrase με λιγότερα σκουπίδια literally means “with fewer pieces of trash” and describes the state of the forest.

Structure:

το δάσος θα είναι πιο καθαρό και με λιγότερα σκουπίδια
= the forest will be cleaner and (will be) with less trash.

This με + noun construction is common in Greek to express a characteristic or accompanying feature of a state:

Το δωμάτιο είναι πιο όμορφο με λουλούδια.
The room is prettier with flowers.
Η πόλη θα είναι πιο ήσυχη, με λιγότερα αυτοκίνητα.
The city will be quieter, with fewer cars.

You could say something like:

…το δάσος θα έχει λιγότερα σκουπίδια.
the forest will have less trash.

This is also correct, but it slightly shifts the focus from the state of the forest to what it has. The original sentence presents “being cleaner and with less trash” as two aspects of the forest’s future condition.

Could you omit με and just say το δάσος θα είναι πιο καθαρό και λιγότερα σκουπίδια?

No, you cannot drop με here.

Without με, λιγότερα σκουπίδια would not be properly linked to the verb θα είναι, so the sentence would be ungrammatical.

You need some connector (like με) to show that λιγότερα σκουπίδια is a characteristic of the state of the forest:

…θα είναι πιο καθαρό και με λιγότερα σκουπίδια. ✔
…θα είναι πιο καθαρό και λιγότερα σκουπίδια. ✘

Why is σκουπίδια plural? Is there a singular form?

Yes, there is a singular:

το σκουπίδι = one piece of trash, one scrap
τα σκουπίδια = trash, garbage, rubbish (in general)

In practice:

For “trash” as a general mass, Greek almost always uses the plural τα σκουπίδια, even if English uses an uncountable singular (trash, rubbish, garbage).
The singular σκουπίδι is used when you explicitly mean one item: “pick up that piece of trash.”

So λιγότερα σκουπίδια = less trash / fewer pieces of rubbish, using the natural plural form.

Why is there a comma after Όταν τελειώσει η εκδρομή?

In Greek punctuation, when a subordinate clause (like an όταν-clause) comes before the main clause, you normally separate them with a comma:

Όταν τελειώσει η εκδρομή, το δάσος θα είναι…

If you reverse the order, the comma usually disappears:

Το δάσος θα είναι πιο καθαρό και με λιγότερα σκουπίδια όταν τελειώσει η εκδρομή.

So the comma marks the end of the introductory “when…” clause.

What exactly does Όταν mean here, and is it always accented like that?

Όταν means “when” (introducing a time clause).

It is always written with an accent: όταν (or Όταν at the beginning of a sentence).
It introduces a time clause that can refer to:
- the present: Όταν έχω χρόνο, διαβάζω. – When I have time, I read.
- the future: Όταν τελειώσει η εκδρομή, θα φύγουμε. – When the trip is over, we’ll leave.

When it refers to the future, it is normally followed by the subjunctive (as in this sentence).