Learn Hardcore Greek: Τον χειμώνα μένω συχνά σπίτι το βράδυ. - In winter I often stay at home in the evening.

In this sentence, Τον χειμώνα literally means the winter, but in Greek, the accusative with a time word often means during / in [that time period].

So:

Τον χειμώνα μένω συχνά σπίτι το βράδυ.
≈ (During) the winter I often stay home in the evening.

This is an example of the accusative of time. You will see similar things like:

Τη Δευτέρα = on Monday
Το πρωί = in the morning
Κάθε μέρα (no article here, but same idea of time expression)

English does not usually use a definite article here, but Greek often does, especially with seasons and days when you mean in that period generally:

Το καλοκαίρι = in (the) summer
Τον χειμώνα = in (the) winter

Why is Τον χειμώνα in the accusative case? I thought subjects were in the nominative.

You are right that subjects are in the nominative, but Τον χειμώνα is not the subject here. The subject is (εγώ), which is just implied by the verb μένω (I stay).

Τον χειμώνα is an adverbial phrase of time: it tells us when the action happens. Greek often uses the accusative case to express duration or time when.

Examples:

Την Κυριακή δουλεύω. = I work on Sunday.
Κάθε βράδυ διαβάζω. = I read every evening.
Τον χειμώνα κάνει κρύο. = In (the) winter it is cold.

So Τον χειμώνα is accusative because of its function (time expression), not because it is the subject.

In English we say in the winter, but in Greek there is no separate word for in here. Why is that?

Greek often expresses “in + time period” using just the article + noun in the accusative, with no preposition.

So:

Τον χειμώνα = in (the) winter
Το καλοκαίρι = in (the) summer
Το βράδυ = in the evening / at night
Τη Δευτέρα = on Monday

You can sometimes use prepositions with time (e.g. σε δύο μέρες = in two days), but for seasons, parts of the day, and days of the week used generally, Greek very often uses the bare accusative without a preposition.

Why is it μένω σπίτι and not μένω στο σπίτι? What is the difference?

Both are correct, but there is a nuance:

μένω σπίτι
Literally: I stay (at) home
This is a more general, idiomatic expression meaning I stay home (not going out, not going elsewhere). No article, no preposition.
μένω στο σπίτι
Literally: I stay in the house / at the house
This is more concrete, referring to a specific house or place.

Some typical patterns:

Πάω σπίτι. = I go home.
Είμαι σπίτι. = I am at home.
Μένω σπίτι σήμερα. = I’m staying home today.

But:

Είμαι στο σπίτι των γονιών μου. = I’m at my parents’ house.
Μένω στο καινούργιο μου σπίτι. = I live in my new house.

So σπίτι without article behaves almost like an adverb meaning at home, while το σπίτι / στο σπίτι is the house / at the house as a physical place.

Why doesn’t σπίτι have an article here, while τον χειμώνα and το βράδυ do?

The noun σπίτι is special in Greek: without an article, it often functions like home in English, almost as an adverb:

Πάω σπίτι. = I’m going home.
Είμαι σπίτι. = I’m (at) home.
Μένω σπίτι. = I stay home.

When you add an article, it becomes a more specific house:

Πάω στο σπίτι. = I’m going to the house.
Είμαι στο σπίτι μου. = I’m at my house.
Μένω στο σπίτι των φίλων μου. = I stay at my friends’ house.

For τον χειμώνα and το βράδυ, the article is part of the normal way Greek talks about time periods in a general sense:

Τον χειμώνα = (in) winter
Το βράδυ = (in) the evening

So:

No article with σπίτι here because it has developed a special idiomatic use meaning home.
Article with χειμώνας, βράδυ because this is the standard way to express in winter / in the evening.

Where does the adverb συχνά usually go? Could I say μένω σπίτι συχνά instead?

Adverbs like συχνά (often) are fairly flexible in Greek, but they most commonly appear:

After the verb, before the rest of the phrase.

So the original:

Τον χειμώνα μένω συχνά σπίτι το βράδυ.

is very natural.

Other acceptable positions:

Τον χειμώνα συχνά μένω σπίτι το βράδυ.
Τον χειμώνα μένω σπίτι συχνά το βράδυ. (less typical, but possible for emphasis)

Μένω σπίτι συχνά is grammatically okay, but in a full sentence with more elements, most speakers would prefer μένω συχνά σπίτι.

General guideline:
Place συχνά close to the verb (μένω) and before the object or place (σπίτι) unless you have a specific reason to emphasize something else.

Could I change the word order to Συχνά μένω σπίτι τον χειμώνα το βράδυ? Would that sound natural?

Yes, Greek word order is quite flexible, and Συχνά μένω σπίτι τον χειμώνα το βράδυ is grammatically correct.

However, emphasis changes slightly:

Τον χειμώνα μένω συχνά σπίτι το βράδυ.
– First focus: in winter; then we learn what happens then.
Συχνά μένω σπίτι τον χειμώνα το βράδυ.
– First focus: often; we start by talking about the frequency.

All of these are possible:

Τον χειμώνα μένω συχνά σπίτι το βράδυ.
Συχνά τον χειμώνα μένω σπίτι το βράδυ.
Τον χειμώνα συχνά μένω σπίτι το βράδυ.

The original order (Τον χειμώνα μένω συχνά σπίτι το βράδυ) is very neutral and natural, so it is a good model to copy.

Why is the present tense μένω used to talk about a general habit? Shouldn’t it be some kind of “habitual tense”?

In Greek, the simple present is the normal tense for:

Habits and repeated actions
General truths
Ongoing present actions (depending on context)

Here, μένω means I stay but the context (time expressions like Τον χειμώνα, συχνά, το βράδυ) makes it clear this is a habit, not a one‑time action.

So:

Τον χειμώνα μένω συχνά σπίτι το βράδυ.
= In winter, I often stay home in the evening. (habit)

Other examples:

Κάθε μέρα πίνω καφέ. = Every day I drink coffee.
Το καλοκαίρι κολυμπάω στη θάλασσα. = In summer I swim in the sea.

There is no special separate tense for habits; the present tense plus context does the job.

What exactly does το βράδυ mean here? Is it at night or in the evening? And why the article το?

Το βράδυ literally means the evening / the night (as a period of time). In English translation it will usually be:

in the evening, or sometimes
at night, depending on context.

In this sentence, it is best understood as in the evening.

The article το is there for the same reason as in Τον χειμώνα:

Greek often uses article + time word in the accusative to mean in / at that time (generally).

Compare:

Το πρωί πίνω καφέ. = In the morning I drink coffee.
Το βράδυ βλέπω τηλεόραση. = In the evening I watch TV.

So:

το βράδυ (accusative with article) = in the evening (as a general time slot)

Could I say τα βράδια instead of το βράδυ? What would be the difference?

Yes, you could say:

Τον χειμώνα μένω συχνά σπίτι τα βράδια.

Both το βράδυ and τα βράδια are possible, but:

το βράδυ (singular)
→ more like in the evenings / in the evening as a time of day in general.
τα βράδια (plural)
→ more explicitly in the evenings, emphasizing repeated evenings, night after night.

The meaning is very close in this sentence, but τα βράδια slightly highlights the repetition of individual evenings.

Could I replace μένω with κάθομαι here? What is the difference between μένω σπίτι and κάθομαι σπίτι?

You can say:

Τον χειμώνα κάθομαι συχνά σπίτι το βράδυ.

It is understandable and acceptable, but there is a nuance:

μένω σπίτι
→ more neutral: I stay home (I do not go out). It focuses on the fact of staying at home.
κάθομαι σπίτι
→ tends to suggest hanging around / just staying in, sometimes with a nuance of sitting around, not doing much, or not going anywhere.

In many everyday contexts, they can overlap:

Σήμερα θα μείνω σπίτι. = I’ll stay home today.
Σήμερα θα κάτσω σπίτι. = I’ll stay in today. (a bit more colloquial)

In your original sentence, μένω σπίτι is the most straightforward and neutral choice.