Learn Hardcore Greek: Συνήθως ψωνίζω σε ένα μεγάλο πολυκατάστημα που έχω προγραμματίσει να πηγαίνω κάθε μήνα. - I usually shop in a big department store that I have planned to visit every month.

All of these are grammatically correct. Changing the position mainly affects rhythm and slight emphasis, but not the basic meaning. Sentence‑initial Συνήθως is the default choice.

What is the nuance of ψωνίζω here? How is it different from αγοράζω?

Both can be translated as to buy, but they’re used differently:

ψωνίζω = to shop, to do one’s shopping.
It often refers to the activity of shopping in general, especially for food, clothes, etc.
- Σήμερα θα ψωνίσω στο σούπερ μάρκετ. = I’ll do my shopping at the supermarket today.
- It can take an object too: ψωνίζω ρούχα (I shop for clothes).
αγοράζω = to buy (a specific thing).
It focuses on the actual act of purchasing something:
- Αγόρασα ένα πουκάμισο. = I bought a shirt.

Preposition patterns are different too:

ψωνίζω σε ένα πολυκατάστημα = I shop at a department store.
αγοράζω από ένα πολυκατάστημα = I buy (something) from a department store.

So here ψωνίζω σε ένα μεγάλο πολυκατάστημα sounds natural and idiomatic.

Why is it σε ένα μεγάλο πολυκατάστημα and not using στο?

σε plus a definite article contracts, but with an indefinite article it doesn’t:

σε + το → στο
σε + τα → στα
σε + τον → στον

So:

σε το πολυκατάστημα → στο πολυκατάστημα (at the department store – specific one)

But with an indefinite article ένα, there is nothing to contract:

σε ένα πολυκατάστημα (at a department store)

In spoken Greek you’ll very often hear σ’ ένα πολυκατάστημα (elision of the vowel), and you can also write it that way: σ’ ένα. Both σε ένα and σ’ ένα are correct in writing; σ’ ένα looks a bit more colloquial.

What gender is πολυκατάστημα, and how do the article and adjective agree with it?

πολυκατάστημα is neuter.

In the phrase σε ένα μεγάλο πολυκατάστημα:

ένα = neuter singular accusative (indefinite article)
μεγάλο = neuter singular accusative (adjective)
πολυκατάστημα = neuter singular accusative (noun)

All three have to match in:

gender (neuter),
number (singular),
case (accusative, because of σε).

Basic forms of πολυκατάστημα:

το πολυκατάστημα (nom./acc. sg.)
του πολυκαταστήματος (gen. sg.)
τα πολυκαταστήματα (nom./acc. pl.)
των πολυκαταστημάτων (gen. pl.)

What exactly does που refer to in this sentence? Is it like “that”, “which”, or “where”? Could we say όπου or στο οποίο instead?

Here που is a relative pronoun/particle that refers back to πολυκατάστημα. The English equivalent would be something like:

“…a big department store (that / which / where) I have planned to go every month.”

Modern Greek uses που very widely for “that / who / which / where” in relative clauses:

Ο φίλος που είδα χθες… = The friend (whom) I saw yesterday…
Το μέρος που μένω… = The place where I live…

You could make it more explicit/formal:

…σε ένα μεγάλο πολυκατάστημα, στο οποίο έχω προγραμματίσει να πηγαίνω κάθε μήνα.
Or: …σε ένα μεγάλο πολυκατάστημα όπου έχω προγραμματίσει να πηγαίνω κάθε μήνα.

Differences:

που is the normal, everyday choice.
όπου highlights the idea of “where” a bit more.
στο οποίο is more formal/written and very clear about the preposition (σε).

In everyday speech, που is by far the most common.

Why is it έχω προγραμματίσει and not just a simple past like προγραμμάτισα?

Έχω προγραμματίσει is the present perfect in Greek: έχω + perfective stem προγραμματίσει.

έχω προγραμματίσει = I have planned / I have scheduled

It emphasizes:

that the planning was done in the past, and
that its result is still relevant now (there is an ongoing plan).

Contrast:

Προγραμμάτισα να πηγαίνω κάθε μήνα.
= I planned to go every month (simple past, the event of planning itself, more detached).

Using έχω προγραμματίσει suggests: I already have this plan in place; it still applies now. In everyday language, Greeks might also say:

που έχω κανονίσει να πηγαίνω κάθε μήνα
που έχω σκοπό να πηγαίνω κάθε μήνα

with a very similar meaning.

Why is it να πηγαίνω and not να πάω after έχω προγραμματίσει?

This is about aspect (ongoing vs single action).

να πηγαίνω = imperfective subjunctive → repeated / ongoing action
να πάω = perfective subjunctive → single, whole action

Here the plan is to go every month, i.e. regularly and repeatedly, so the imperfective form makes sense:

…έχω προγραμματίσει να πηγαίνω κάθε μήνα.
= I have planned to go (regularly) every month.

Compare:

Θέλω να πάω στο Λονδίνο.
= I want to go to London (once).
Θέλω να πηγαίνω στο γυμναστήριο κάθε μέρα.
= I want to go to the gym every day (repeatedly).

So να πηγαίνω matches the idea of a recurring visit.

Is πηγαίνω different from πάω? They both seem to mean “I go”.

Yes, they are related forms of the same verb with different aspects:

πηγαίνω is the imperfective stem (ongoing, repeated).
πάω is the perfective stem in many forms, but in modern usage it also acts as a present form.

Some indicative forms:

Present: πηγαίνω / (colloquial) πάω
Future (imperfective): θα πηγαίνω (I will be going / I will go regularly)
Future (perfective): θα πάω (I will go once)
Past (perfective): πήγα (I went)
Past (imperfective): πήγαινα (I was going / used to go)
Subjunctive (imperfective): να πηγαίνω
Subjunctive (perfective): να πάω

In practice:

For one single trip: Θέλω να πάω στην τράπεζα.
For a habit: Θέλω να πηγαίνω στην τράπεζα κάθε Δευτέρα.

In your sentence, να πηγαίνω is chosen for the habitual meaning.

What role does σε ένα μεγάλο πολυκατάστημα play in the sentence? Why is it accusative after σε?

σε ένα μεγάλο πολυκατάστημα is a prepositional phrase indicating place: at a big department store.

In Greek, the preposition σε always takes the accusative case:

σε έναν φίλο (to a friend)
σε ένα δωμάτιο (in a room)
σε μια πόλη (in a city)

So πολυκατάστημα is in the accusative because it’s the object of the preposition σε, not because it’s a direct object of the verb. The whole phrase σε ένα μεγάλο πολυκατάστημα functions as an adverbial of place modifying ψωνίζω.

Can we omit ένα and just say σε μεγάλο πολυκατάστημα? Would that change the meaning?

You can say σε μεγάλο πολυκατάστημα, and it is grammatically correct. The nuance is slightly different:

σε ένα μεγάλο πολυκατάστημα
= at a big department store (one particular, though not specified which one). It feels like you have a specific store in mind.
σε μεγάλο πολυκατάστημα
= at (some) big department store / at big department stores in general. This sounds a bit more generic or less focused on one particular store.

In your sentence, because you continue with που έχω προγραμματίσει να πηγαίνω κάθε μήνα, the version with ένα fits well: it suggests there is one particular store you have made this monthly plan with.

Can κάθε μήνα appear earlier in the sentence, or must it be at the end?

κάθε μήνα is an adverbial phrase of time (every month), and Greek word order is flexible. You can move it:

Συνήθως ψωνίζω κάθε μήνα σε ένα μεγάλο πολυκατάστημα που έχω προγραμματίσει να πηγαίνω.
Συνήθως κάθε μήνα ψωνίζω σε ένα μεγάλο πολυκατάστημα που έχω προγραμματίσει να πηγαίνω.

Your original version:

…που έχω προγραμματίσει να πηγαίνω κάθε μήνα.

keeps κάθε μήνα close to να πηγαίνω, clearly showing how often you go. Other positions are still understandable, but putting it right after πηγαίνω is very natural and clear for frequency.

Is ψωνίζω σε the only option, or could we also say ψωνίζω από a store?

Both ψωνίζω σε and ψωνίζω από exist, but they’re used a bit differently:

ψωνίζω σε ένα πολυκατάστημα
= I shop at a department store (focus on where you do your shopping).
ψωνίζω από ένα πολυκατάστημα
= I shop from a department store (focus slightly more on the source of the goods).

In everyday speech, for “I (usually) shop at X”, ψωνίζω σε + place is very common and natural, as in your sentence.