Learn Hardcore Greek: Στο πάρτι τη βλέπω δίπλα στο παράθυρο. - At the party I see her next to the window.

Putting it after the verb (βλέπω τη) is not normal. Pronouns can go after the verb only in specific cases (imperatives, some infinitival-like forms, etc.), for example:

Δες τη! – Look at her!
Θέλω να τη δω. – I want to see her. (here it’s still before the verb δω, but after να)

So in your sentence, τη βλέπω is the grammatically correct and natural order.

Why is it τη and not την?

The full form is την, but in everyday Modern Greek the final -ν is often dropped in front of many consonants.

A common rule: keep the final -ν before

vowels (α, ε, η, ι, ο, υ, ω)
and the consonants κ, π, τ, ξ, ψ, γκ, μπ, ντ, τσ, τζ

In front of other consonants, the ν is usually dropped in writing and speech.

Since βλέπω starts with β, the ν can be dropped → τη βλέπω.

Both την βλέπω and τη βλέπω are understandable, but τη βλέπω is the standard modern spelling in this context.

What exactly is τη grammatically?

Τη is:

a third-person singular
feminine
accusative
unstressed object pronoun (clitic)

It corresponds to English “her” as a direct object:

Τη βλέπω. – I see her.
Τη βοηθάω. – I help her.
Τη θυμάμαι. – I remember her.

It refers to a specific feminine person already known from the context or mentioned earlier.

Why not say βλέπω αυτήν instead of τη βλέπω?

You can say βλέπω αυτήν, but it sounds emphatic, like:

Βλέπω αυτήν (και όχι κάποια άλλη).
I see her (this one), not someone else.

In normal, neutral speech, Greek prefers the unstressed clitic τη:

Τη βλέπω δίπλα στο παράθυρο. – I see her next to the window.

Use αυτήν (or αυτή) when you want to emphasize or contrast:

Αυτήν βλέπω, όχι την αδερφή της.
It’s her I see, not her sister.

Why does the sentence start with Στο πάρτι instead of Τη βλέπω στο πάρτι?

Greek word order is flexible, and you can say:

Στο πάρτι τη βλέπω δίπλα στο παράθυρο.
Τη βλέπω στο πάρτι δίπλα στο παράθυρο.

Both are correct. Starting with Στο πάρτι puts emphasis on the setting/time:

Στο πάρτι (not somewhere else) τη βλέπω δίπλα στο παράθυρο.

It’s like saying in English:

At the party, I see her next to the window.

So fronting Στο πάρτι is mainly for focus and style, not grammar.

What tense/aspect is βλέπω, and does it mean “I see” or “I am seeing”?

Βλέπω is:

present tense
imperfective aspect

It can correspond to:

I see her (simple present)
I am seeing her (present continuous)

Modern Greek does not distinguish these two forms as clearly as English does. Context decides whether it’s:

a current ongoing perception – Right now, at the party, I see her there
or a habitual situation – Whenever I’m at the party, I (usually) see her next to the window (less likely here, but possible from context).

Why is there no separate verb meaning “she is next to the window”? How does δίπλα στο παράθυρο work?

In this sentence, δίπλα στο παράθυρο is an adverbial phrase of place modifying βλέπω:

Τη βλέπω δίπλα στο παράθυρο.
I see her (where?) next to the window.

So δίπλα στο παράθυρο tells you where you see her, not a separate “she is next to the window” clause.

To explicitly say “She is next to the window”, you could say:

Είναι δίπλα στο παράθυρο. – She is next to the window.
Αυτή είναι δίπλα στο παράθυρο. – She is next to the window. (more emphasis)

In your sentence, that meaning is implied through the place phrase combined with βλέπω.

What is the structure and meaning of δίπλα στο παράθυρο?

Δίπλα στο παράθυρο is:

δίπλα – next to / beside
στο – σε + το = at/to the
παράθυρο – window (neuter singular)

So literally: “next to the window”.

Grammatically: δίπλα is an adverb/preposition that usually takes σε + accusative:

δίπλα σε μένα – next to me
δίπλα στο σπίτι – next to the house
δίπλα στο παράθυρο – next to the window

Here, again, σε + το → στο, and παράθυρο is in the accusative, because σε always takes the accusative case.

Why is there a definite article (στο παράθυρο) when English might say just “next to a window”?

Modern Greek uses the definite article much more often than English.

δίπλα στο παράθυρο literally = next to the window

But in context, this can be understood as:

the window that is relevant/visible in this situation
effectively “next to the window” or even just “by the window”

If you really mean “a window, any window”, you might say:

δίπλα σε ένα παράθυρο – next to a window

However, δίπλα στο παράθυρο is the more typical, natural way if we are talking about the known window in that room or space.

Why is παράθυρο in that form? Does the preposition σε always use the accusative?

Yes. The preposition σε in Modern Greek is always followed by the accusative case.

Παράθυρο is a neuter noun:

nominative: το παράθυρο
accusative: το παράθυρο
(same form in both cases)

So in στο παράθυρο:

στο = σε + το (article in accusative)
παράθυρο = accusative singular

Even though nominative and accusative look identical here, you should think of it as accusative, because of the preposition σε.

How do I know that τη refers to a feminine person and not to το παράθυρο or το πάρτι?

Greek object pronouns agree in gender and number with the person or thing they refer to, not with nearby nouns.

Here:

το πάρτι – neuter
το παράθυρο – neuter
τη – feminine singular

So τη must refer to a feminine singular referent mentioned earlier in the conversation (e.g. “Maria”, “your sister”, etc.).

If it referred to something neuter, you would see το instead:

Το βλέπω. – I see it. (neuter thing)
Το παράθυρο το βλέπω. – I see the window.

The fact that it’s τη tells you: “we are talking about some she/her we both know from context.”