Learn Hardcore Greek: Η θέση δίπλα στο παράθυρο είναι η πιο άνετη. - The seat next to the window is the most comfortable.

The first Η (capital) belongs to Η θέση – the seat.
The second η belongs to η πιο άνετη – literally the more comfortable, which in context means the most comfortable.

In Greek, you normally use the article η/ο/το with superlative meanings, so you say:

η πιο άνετη – the most comfortable (feminine)
ο πιο άνετος – the most comfortable (masculine)
το πιο άνετο – the most comfortable (neuter)

So the sentence literally is:
The seat next to the window is the most comfortable (one).

Why do we say η πιο άνετη and not just πιο άνετη for “the most comfortable”?

πιο άνετη by itself means more comfortable (comparative, without specifying “the”). For example:

Αυτή η θέση είναι πιο άνετη από την άλλη.
This seat is more comfortable than the other.

When you want the most comfortable (superlative idea), Greek normally keeps the same structure πιο + adjective, but adds the definite article:

η πιο άνετη – the most comfortable (feminine)
ο πιο άνετος – the most comfortable (masculine)

So η πιο άνετη here really means the most comfortable (seat), even though there is no separate superlative ending on the adjective.

Why is άνετη and not άνετος or άνετο? How does adjective agreement work here?

Greek adjectives agree in gender, number, and case with the noun they describe.

The basic forms of this adjective are:

άνετος – masculine singular (e.g. άνετος καναπές – comfortable sofa)
άνετη – feminine singular (e.g. άνετη καρέκλα – comfortable chair)
άνετο – neuter singular (e.g. άνετο κρεβάτι – comfortable bed)

The noun θέση (seat) is:

feminine
singular
nominative (subject of the verb είναι)

So the adjective must also be:

feminine, singular, nominative → άνετη

Hence:
Η θέση … είναι η πιο άνετη.
The seat … is the most comfortable (one).

What exactly is θέση? Does it always mean “seat”?

Θέση is a feminine noun that can mean:

seat – in a bus, theater, airplane, etc.
- Η θέση μου είναι δίπλα στο παράθυρο. – My seat is by the window.
place / spot – parking place, spot in a row, etc.
- Δεν υπάρχει θέση πάρκινγκ. – There’s no parking space.
position – job position or social position
- Έχει καλή θέση στη δουλειά. – He/she has a good position at work.

In this sentence, it clearly means seat (on a bus, plane, train, etc.), because of δίπλα στο παράθυρο.

Why is it δίπλα στο παράθυρο and not just δίπλα παράθυρο or δίπλα το παράθυρο?

In Greek, δίπλα (“next to”) usually combines with the preposition σε:

δίπλα σε
- noun

When the noun has a definite article, σε + article contracts to one word:

σε το → στο (neuter singular)
σε τη(ν) → στη(ν) (feminine singular)
σε τον → στον (masculine singular)
σε τα → στα (neuter plural), etc.

So:

δίπλα σε το παράθυρο
→ δίπλα στο παράθυρο (correct, natural)

You cannot drop the σε and just say δίπλα το παράθυρο; that is ungrammatical.

Could I also say δίπλα από το παράθυρο instead of δίπλα στο παράθυρο?

Yes, δίπλα από το παράθυρο is grammatically correct and understandable.

Differences in feel:

δίπλα στο παράθυρο is the most common and natural everyday phrase for next to the window.
δίπλα από το παράθυρο is also possible, but από often slightly emphasizes “from the side of” and can sound a bit more spatially descriptive or slightly more formal/less idiomatic here.

In this context (a seat on a bus/train/plane), people overwhelmingly say:

Η θέση δίπλα στο παράθυρο…

What case is παράθυρο in here, and why is it in that case?

Παράθυρο is in the accusative singular form.

The core forms of παράθυρο (neuter) are:

Nominative: το παράθυρο – the window (as subject)
Accusative: το παράθυρο – the window (as object / after many prepositions)
(for neuter singular, nominative and accusative look the same)

After the preposition σε (hidden inside στο), the noun goes into the accusative:

σε + (το) παράθυρο → στο παράθυρο (accusative)

So we get δίπλα στο παράθυρο – next to the window.

Can I change the word order to Η πιο άνετη θέση είναι δίπλα στο παράθυρο? Is that still correct?

Yes, that is perfectly correct Greek:

Η πιο άνετη θέση είναι δίπλα στο παράθυρο.
The most comfortable seat is next to the window.

Both versions are grammatical:

Η θέση δίπλα στο παράθυρο είναι η πιο άνετη.
The seat by the window is the most comfortable.
Η πιο άνετη θέση είναι δίπλα στο παράθυρο.
The most comfortable seat is by the window.

The difference is mainly one of focus:

Version 1 starts from a specific seat (the one by the window) and tells you it is the most comfortable.
Version 2 starts from the most comfortable seat and then tells you where it is.

In English we say “the seat by the window”. Is δίπλα στο παράθυρο the normal way to say this, or is there a closer translation of “by”?

In this context, δίπλα στο παράθυρο is exactly what Greek speakers normally say for “by the window / next to the window” when talking about seats.

You could also theoretically use:

κοντά στο παράθυρο – near the window (not necessarily right next to it)
παρά το παράθυρο – next to the window (more old‑fashioned / literary)

But in everyday modern Greek, for a seat directly next to the window:

θέση δίπλα στο παράθυρο is the standard, idiomatic phrase.

Why is there no word like “that” in “the seat that is by the window”? How is θέση δίπλα στο παράθυρο working grammatically?

In English you often use a relative clause:

the seat that is by the window

Greek can also use a relative pronoun, but it doesn’t need to here. The phrase:

η θέση δίπλα στο παράθυρο

is simply “the seat next to the window”, where δίπλα στο παράθυρο functions as a prepositional phrase modifying the noun (like an adjective phrase). There is no need to say που είναι (“that is”) in between.

If you wanted to be more explicit, you could say:

Η θέση που είναι δίπλα στο παράθυρο είναι η πιο άνετη.
The seat that is next to the window is the most comfortable.

But the shorter original version is more natural and completely clear.

Why is there no subject pronoun like “it” in front of είναι? Shouldn’t it be something like αυτή είναι?

In Greek, subject pronouns (εγώ, εσύ, αυτός, αυτή, αυτό, etc.) are often dropped because the subject is clear from context or from the verb form.

Here, the subject is explicitly present as a noun phrase:

Η θέση δίπλα στο παράθυρο – the seat by the window

So you just say:

Η θέση δίπλα στο παράθυρο είναι η πιο άνετη.

If you added a pronoun, it would sound redundant or marked:

Αυτή η θέση δίπλα στο παράθυρο είναι η πιο άνετη. – This seat by the window is the most comfortable.
(Here the pronoun αυτή is a demonstrative this, not a simple subject pronoun “she/it”.)

So Greek doesn’t need a separate “it” before είναι; the noun phrase is the subject.