Learn Hardcore Greek: Ο γονέας είναι στο σπίτι με το παιδί.

γονέας is grammatically masculine, so it always takes the masculine article ο in the nominative: ο γονέας.
το is the neuter article. You use it with neuter nouns like το σπίτι, το παιδί.

So the article is not chosen by the real-life gender of the person, but by the grammatical gender of the noun.

If the parent is a mother, shouldn’t it be Η γονέας instead of Ο γονέας?

No. Even if you are talking about a mother, the noun γονέας itself is grammatically masculine, so it still uses the masculine article ο:

ο γονέας = the parent (could be mother or father)

If you specifically want to say:

the mother → η μητέρα
the father → ο πατέρας

The word γονέας is a formal, legal-sounding word meaning “parent,” and it keeps masculine agreement in grammar, regardless of the actual person’s sex.

What exactly does είναι mean, and why doesn’t it change for “he / she / it”?

είναι is the present tense of the verb είμαι (“to be”), used for:

he is / she is / it is
they are

So:

αυτός είναι = he is
αυτή είναι = she is
αυτό είναι = it is
αυτοί / αυτές / αυτά είναι = they are

That’s why you see είναι even though γονέας is singular: “The parent is …” uses the 3rd person singular form, which in Greek happens to look the same as the 3rd person plural.

Full present tense for reference (very common verb):

εγώ είμαι – I am
εσύ είσαι – you are (singular)
αυτός/αυτή/αυτό είναι – he/she/it is
εμείς είμαστε – we are
εσείς είστε – you are (plural/polite)
αυτοί/αυτές/αυτά είναι – they are

What is στο in στο σπίτι? Is it one word or two?

στο is a contraction of two words:

σε = in, at, to
το = the (neuter, singular, accusative)

So:

σε + το = στο

Literally, στο σπίτι = “in/at the house,” and in this sentence it is used in the sense of “at home.” Greek very often contracts σε + article:

σε + τον → στον (masc.)
σε + την → στην (fem.)
σε + το → στο (neuter)

Why is it το σπίτι and not ο σπίτι or η σπίτι?

Because σπίτι is a neuter noun.

Definite articles in the singular nominative/accusative are:

ο – masculine
η – feminine
το – neuter

So you get:

το σπίτι – the house / the home
ο γονέας – the parent
το παιδί – the child

Also, neuter nouns have the same form for nominative and accusative, so σπίτι looks the same whether it’s a subject or an object (or after a preposition).

Why does Greek say στο σπίτι (“in/at the house”) when English just says “at home” without the?

Greek uses the definite article much more often than English.

στο σπίτι literally: “in/at the house”
In natural English: “at home” / “at the house”

In Greek, στο σπίτι is the normal way to say “at home.” English omits the and often the word “house,” but Greek keeps both the noun and the article.

You could also say:

είναι σπίτι = he/she is (at) home (more colloquial, without the article)
είναι στο σπίτι του = he is at his house

In the sentence you gave, στο σπίτι is the standard, neutral form: “at home.”

Why is παιδί neuter? Does that mean the child’s gender is unknown?

Many Greek words for young children or babies are grammatically neuter, regardless of the real-life gender:

το παιδί – the child
το μωρό – the baby

So το παιδί does not mean “an ‘it’ child” or “gender unknown” in a special way; it’s just how the language works.

If you need to be specific, you can add a word:

το αγόρι – the boy
το κορίτσι – the girl

Or you make it clear from context: “my son,” “my daughter,” etc.

Why are σπίτι and παιδί in the same form, even though they have different roles in the sentence?

Both σπίτι and παιδί are neuter nouns in the accusative singular:

στο σπίτι – “in/at the house” (σε
- accusative)
με το παιδί – “with the child” (με
- accusative)

Two key points:

The prepositions σε and με take the accusative case.
For neuter nouns, nominative and accusative look the same in the singular (and in the plural too).

So even though σπίτι and παιδί have different roles (one is a location after σε, the other is the object of με), their form is identical because of neuter case endings.

Could the word order change and still mean the same thing?

Yes. Greek word order is fairly flexible. All of these can be correct:

Ο γονέας είναι στο σπίτι με το παιδί.
Ο γονέας με το παιδί είναι στο σπίτι.
Στο σπίτι είναι ο γονέας με το παιδί.

The basic meaning “The parent is at home with the child” stays the same. Different orders can slightly change the emphasis:

Starting with Στο σπίτι… highlights the place.
Grouping ο γονέας με το παιδί together highlights them as a unit.

But grammatically they are all fine.

Can I leave out Ο γονέας and just say Είναι στο σπίτι με το παιδί?

Yes, you can. Greek is a pro‑drop language: subject pronouns (and often even nouns) can be omitted when context makes them clear.

Είναι στο σπίτι με το παιδί.
= He/She/It is at home with the child.

Without ο γονέας, the subject is understood from context (who you’re talking about). In a standalone sentence with no context, Ο γονέας είναι… is clearer because it names the subject explicitly.