Θέλω να καθίσω δίπλα σε ένα παράθυρο.

In Modern Greek, after θέλω (I want), you normally use να plus a verb:

• Θέλω να φύγω = I want to leave
• Θέλω να δω = I want to see
• Θέλω να καθίσω = I want to sit / sit down

So να is the normal marker that introduces the next verb. English uses to in many cases; Greek usually uses να instead.

Without να, Θέλω καθίσω is not correct Greek.

καθίσω is the form meaning to sit down / to take a seat in this sentence.

It comes from the verb κάθομαι (I sit / I am sitting), but Greek uses different stems/forms depending on meaning and aspect.

Here, να καθίσω suggests a single action:

• I want to sit down
• I want to take a seat

It focuses on the act of moving into a sitting position, not on the ongoing state of already being seated.

This is about aspect, which is very important in Greek.

• να καθίσω = to sit down, as a single complete action
• να κάθομαι = to be sitting, to sit habitually or continuously

So:

• Θέλω να καθίσω δίπλα σε ένα παράθυρο = I want to sit down next to a window
• Θέλω να κάθομαι δίπλα στο παράθυρο could mean something more like I want to be sitting next to the window / I like to sit next to the window

In this sentence, the speaker is probably choosing a seat, so καθίσω is the natural choice.

Yes. Να κάτσω is very common in everyday spoken Greek.

So these are both natural:

• Θέλω να καθίσω δίπλα σε ένα παράθυρο
• Θέλω να κάτσω δίπλα σε ένα παράθυρο

Very roughly:

• καθίσω can sound a bit more standard or careful
• κάτσω is extremely common in speech

A learner should understand both.

δίπλα σε means next to, beside, or by.

It works as a fixed expression:

• δίπλα σε έναν φίλο = next to a friend
• δίπλα σε μια πόρτα = next to a door
• δίπλα σε ένα παράθυρο = next to a window

You should learn δίπλα σε together as a unit.

Because ένα παράθυρο means a window, while το παράθυρο means the window.

So:

• σε ένα παράθυρο = next to a window
• στο παράθυρο = next to the window / at the window

Also, στο is just the contraction of σε + το:

• σε το → στο

But there is no similar contraction with ένα, so it stays:

• σε ένα

Because παράθυρο is a neuter noun.

Greek articles must match the noun’s gender:

• masculine: ένας / έναν
• feminine: μια / μία
• neuter: ένα

So:

• ένα παράθυρο = a window

Other examples:

• ένας άνθρωπος = a man / a person
• μια καρέκλα = a chair
• ένα τραπέζι = a table

Since παράθυρο is neuter, ένα is the correct form.

It is in the accusative after σε.

However, with many neuter nouns, the nominative and accusative forms look the same, so you do not see a change:

• nominative: το παράθυρο
• accusative: το παράθυρο

So even though the case is accusative after σε, the form still appears as παράθυρο.

You can see the case more clearly with masculine nouns:

• δίπλα σε ένας φίλος ❌
• δίπλα σε έναν φίλο ✅

The given word order is natural and neutral:

• Θέλω να καθίσω δίπλα σε ένα παράθυρο.

Greek word order is more flexible than English, so you may hear variations for emphasis, such as:

• Δίπλα σε ένα παράθυρο θέλω να καθίσω.

This puts more emphasis on next to a window.

Still, for a learner, the original order is the safest and most natural default.

A simple pronunciation guide is:

THÉ-lo na ka-THÍ-so DÍ-pla se É-na pa-RÁ-thi-ro

A few notes:

• θ sounds like th in think
• δ sounds like th in this
• χ does not appear here, but learners often confuse it with θ, so it is worth keeping them separate
• Stress matters in Greek, and the stressed syllables are:
  • Θέλω
  • καθίσω
  • δίπλα
  • ένα
  • παράθυρο

Its basic meaning is the physical window, and that is exactly what it means here.

Like English, Greek can also use παράθυρο metaphorically in some contexts, for example an opportunity or an opening, but in this sentence it simply means a real window in a room, vehicle, etc.

So a learner should understand this sentence very literally: I want to sit next to a window.

Yes, very natural.

You could say it when asking for a seat preference:

• on a plane
• on a train
• on a bus
• in a café or restaurant

If you want to sound especially polite, you might also hear:

• Θα ήθελα να καθίσω δίπλα σε ένα παράθυρο.
• Μπορώ να καθίσω δίπλα σε ένα παράθυρο;

These mean:

• I would like to sit next to a window
• Can I sit next to a window?

But the original sentence is completely correct and natural.