Learn Hardcore Greek: Σήμερα είμαι στο σπίτι όλη τη μέρα.

είμαι στο σπίτι = I am at the house / at home (more explicit, slightly more neutral)
είμαι σπίτι = I’m home (more colloquial, feels very close to English “I’m home”)

In your sentence, στο σπίτι is perfectly standard and clear: I’m at home.

What exactly is στο? Why is it one word?

στο is a contraction of the preposition σε (in/at/on) and the neuter article το (the):

σε + το σπίτι → στο σπίτι

This type of contraction is very common:

σε + τον δρόμο → στον δρόμο (in/on the street)
σε + την πόλη → στην πόλη (in the city)

So you write and pronounce it as στο, not σε το.

What case is σπίτι in, and why, if it means a location?

σπίτι here is in the accusative singular neuter (form: το σπίτι / στο σπίτι).

In modern Greek, after the preposition σε (“in, at, on”), the noun normally goes in the accusative, even when it expresses location:

στο σπίτι = at home
στην Αθήνα = in Athens
στο γραφείο = at the office

So although in English we don’t think of “at home” as an “object,” Greek still uses the accusative after σε.

Why is it όλη τη μέρα and not just όλη μέρα?

Both exist and are very common:

όλη τη μέρα = literally “all the day”
όλη μέρα = “all day” (without the article)

Your sentence uses the version with the article:

όλη (all/whole) – feminine singular, agreeing with μέρα
τη – feminine accusative article “the”
μέρα – “day,” in the accusative (object-like form)

Meaning-wise, είμαι στο σπίτι όλη τη μέρα ≈ είμαι στο σπίτι όλη μέρα → I’m at home all day.
The difference is tiny in practice; both are natural.

Why is it τη μέρα and not την μέρα?

The full feminine accusative article is την. In modern usage, the final -ν is often dropped before many consonants in writing:

την ημέρα (before a vowel → we normally keep the ν)
τη μέρα (before the consonant μ, the ν is commonly dropped)

So:

τη μέρα and την μέρα are both understood.
In everyday writing you will very often see τη μέρα in this sentence.
Pronunciation is normally ti either way; the ν is weak and often not clearly heard.

Why is όλη in the feminine form here?

Because adjectives in Greek agree with the noun they describe in:

gender
number
case

The noun μέρα is:

feminine
singular
accusative

So όλη must also be:

feminine
singular
accusative → όλη

If the noun were neuter or masculine, όλη would change:

όλο το βράδυ = all evening (neuter)
όλο τον μήνα = all month (masculine)

Why is there no word for “I” (like εγώ) in the sentence?

Greek usually drops subject pronouns because the verb ending already shows the person:

είμαι = I am
είσαι = you are
είναι = he/she/it is or they are (depending on context)

So είμαι already tells you “I”. Adding εγώ is only needed for emphasis or contrast:

Σήμερα είμαι στο σπίτι όλη τη μέρα. = Today I’m at home all day.
Σήμερα εγώ είμαι στο σπίτι, όχι εσύ. = I am at home today, not you.

In your neutral sentence, omitting εγώ is normal and more natural.

Can the word order change? For example, can I say Είμαι στο σπίτι σήμερα όλη τη μέρα?

Yes. Greek word order is quite flexible. All of these are possible and natural:

Σήμερα είμαι στο σπίτι όλη τη μέρα.
Είμαι στο σπίτι όλη τη μέρα σήμερα.
Σήμερα όλη τη μέρα είμαι στο σπίτι.

The differences are mainly in emphasis and rhythm:

Putting Σήμερα at the beginning makes “today” the topic.
Moving σήμερα later (e.g. …όλη τη μέρα σήμερα) slightly emphasizes “all day” more.

For an everyday neutral statement, your original order is very typical.

Why is the present tense είμαι used, even though “all day” includes the future part of the day?

In Greek, the present tense is often used for:

What is already true now and will continue
- Σήμερα είμαι στο σπίτι όλη τη μέρα.
  → I’m at home (now) and I’ll stay all day.
Near-future plans that feel arranged or fixed, especially within the same day
Similar to English “I’m staying home all day today.”

If you wanted to emphasize a future plan more clearly, you could say:

Σήμερα θα είμαι στο σπίτι όλη τη μέρα.
(Today I will be at home all day.)

But Σήμερα είμαι στο σπίτι όλη τη μέρα already works very naturally for “I’m at home all day today.”

Is there a difference between σπίτι and στο σπίτι for “home”?

Yes, a small nuance:

είμαι σπίτι
→ I’m home (colloquial, very common in speech; no article, feels less “physical/place” and more “at home” as a state).
είμαι στο σπίτι
→ I’m at the house / at home (slightly more explicit and neutral; emphasizes the place a bit more).

In many contexts they can be used interchangeably, but:

To say where you live, you’d usually say μένω στο σπίτι (I live in the house) or μένω σε αυτό το σπίτι (I live in this house), not μένω σπίτι.
To answer the phone and say where you are right now, είμαι σπίτι is very natural.

How do you pronounce Σήμερα είμαι στο σπίτι όλη τη μέρα?

Approximate pronunciation (stressed syllables in CAPITALS):

ΣΗ-με-ρα → SÍ-me-ra
ΕΙ-μαι → Í-me
στο → sto
ΣΠΙ-τι → SPÍ-ti
Ο-λη → Ó-li
τη → ti
ΜΕ-ρα → MÉ-ra

Spoken smoothly: SÍmera Íme sto SPÍti Óli ti MÉra.

Key points:

ει in είμαι is pronounced like i (as in machine), not “ee-eh.”
Double consonants like π
- τ in σπίτι don’t lengthen; it’s just SPÍ-ti.
Each written vowel is its own syllable; Greek spelling is quite phonetic.

What’s the difference between μέρα and ημέρα?

They are essentially the same word, but:

μέρα = everyday, colloquial form (what you’ll usually say in speech)
ημέρα = more formal or written form (common in official language, documents, announcements)

So you could say:

Σήμερα είμαι στο σπίτι όλη τη μέρα. (most natural in conversation)
Σήμερα είμαι στο σπίτι όλη την ημέρα. (a bit more formal)

Meaning in both cases: I’m at home all day today.