Χρειάζομαι μια λύση που να είναι ουσιαστική και όχι απλώς κάτι περιττό.

Χρειάζομαι is the normal modern Greek verb meaning I need.

A few useful points:

• It is a deponent verb, meaning it has a middle/passive-looking form but an active meaning.
• So even though it ends in -ομαι, it does not mean I am being needed here. It simply means I need.

Examples:

• Χρειάζομαι βοήθεια. = I need help.
• Χρειάζομαι χρόνο. = I need time.

So in this sentence:

• Χρειάζομαι μια λύση = I need a solution

μια λύση is in the accusative singular, because it is the direct object of Χρειάζομαι.

Breakdown:

• μια = a / one (feminine singular)
• λύση = solution (feminine noun)

Because λύση is feminine singular, the article/indefinite article must match:

• μια λύση = a solution

This is one of those cases where the noun form looks the same as the nominative, but functionally it is the object of the verb.

This is a very common learner question, and it is important.

που να είναι here means something like:

• that would be
• that is to be
• that should be
• that be in a more abstract English sense

In Greek, που + να + verb is often used after an indefinite or desired noun to describe the kind of thing wanted, not a specific already-identified thing.

So:

• μια λύση που είναι ουσιαστική = a solution that is substantial
  This sounds more like you are talking about a solution already known or being identified.

• μια λύση που να είναι ουσιαστική = a solution that is substantial / that will be substantial / that actually be meaningful
  This emphasizes the desired quality of the solution.

Since the speaker is saying what kind of solution they need, που να είναι is the natural choice.

να είναι is the subjunctive construction of είμαι (to be).

• είναι = he/she/it is
• να είναι = to be / be / should be, depending on context

Greek does not have an infinitive like English to be in the same way, so να + finite verb often does that job.

In this sentence:

• που να είναι ουσιαστική = that is/should be meaningful/substantial

It expresses a desired characteristic, not just a plain statement of fact.

Because it agrees with λύση, which is a feminine singular noun.

• η λύση = the solution (feminine)
• therefore:
  • ουσιαστική λύση = substantial/meaningful solution

Greek adjectives must agree with the noun in:

• gender
• number
• case

So here:

• μια λύση → feminine singular
• ουσιαστική → feminine singular

It can mean substantial, but in this sentence it is probably closer to:

• meaningful
• real
• effective
• genuine
• of real substance

The adjective comes from ουσία, meaning essence or substance.

So ουσιαστική suggests something that has real value or real impact, not something superficial.

In this sentence:

• μια λύση που να είναι ουσιαστική means a solution that is actually meaningful/effective, not just something that looks like a solution.

Because όχι is used here to negate a phrase or contrast an idea, not directly to negate a finite verb.

Compare:

• δεν είναι καλό = it is not good
  Here δεν negates the verb.

• καλό και όχι ακριβό = good and not expensive
  Here όχι contrasts one description with another.

In your sentence:

• ουσιαστική και όχι απλώς κάτι περιττό = substantial/meaningful and not merely something unnecessary

So όχι is the correct choice because it is setting up a contrast:

• not X, but Y or here:
• Y, and not merely X

απλώς means:

• simply
• just
• merely

In this sentence, it softens and sharpens the contrast at the same time:

• όχι απλώς κάτι περιττό = not merely something unnecessary

So the speaker is not rejecting just any solution; they are saying they want something truly useful, not just some extra thing with no real point.

περιττό means:

• unnecessary
• superfluous
• redundant
• extra in a pointless way

It is neuter singular here because it goes with κάτι.

• κάτι = something
• after κάτι, adjectives are usually in the neuter singular

So:

• κάτι περιττό = something unnecessary / something superfluous

Even though English does not show this kind of agreement, Greek does.

Because κάτι already means something, so it works like an indefinite pronoun by itself.

• κάτι = something
• κάτι περιττό = something unnecessary

You would not normally add an article there.

It functions as a complete noun phrase on its own.

Yes, Greek word order is fairly flexible, but the original order is natural and clear.

Original:

• Χρειάζομαι μια λύση που να είναι ουσιαστική και όχι απλώς κάτι περιττό.

This flows like:

1. I need
2. a solution
3. that be meaningful
4. and not merely something unnecessary

The order places the emphasis naturally on the qualities of the solution.

A different order might still be grammatical, but could sound more marked or less natural. For learners, the original word order is a good model.

Yes. The whole relative clause describes μια λύση.

Structure:

• Χρειάζομαι = I need
• μια λύση = a solution
• που να είναι ουσιαστική και όχι απλώς κάτι περιττό = that be meaningful and not merely something unnecessary

So the speaker is not saying:

• I need a solution, and not just something unnecessary

They are specifically saying:

• I need a solution of the kind that is meaningful and not merely superfluous

Because once λύση has already been mentioned, Greek does not need to repeat it.

The clause:

• που να είναι ουσιαστική literally means:
• that be substantial/meaningful

The adjective ουσιαστική is enough because it clearly refers back to λύση.

English does the same:

• I need a solution that is meaningful not
• I need a solution that is a meaningful solution

So Greek is behaving very naturally here.

Yes — absolutely. It is a very useful pattern.

A good general idea is:

• noun + που να + verb
  = noun that should / would / is meant to ... = a noun with a desired or expected characteristic

Examples:

• Θέλω κάποιον που να με καταλαβαίνει.
  = I want someone who understands me.

• Ψάχνω ένα σπίτι που να είναι ήσυχο.
  = I’m looking for a house that is quiet.

• Χρειάζομαι μια λύση που να δουλεύει.
  = I need a solution that works.

So your sentence is a very typical and useful Greek structure.