Learn Hardcore Greek: Όταν είμαι μόνος στο σπίτι τη νύχτα, κλειδώνω την πόρτα. - When I am alone at home at night, I lock the door.

Όταν means when and introduces a time clause. So Όταν είμαι μόνος στο σπίτι τη νύχτα sets the situation/time, and the main clause κλειδώνω την πόρτα gives what happens in that situation.

Why is there a comma after τη νύχτα?

In Greek, it’s very common to use a comma after a fronted subordinate clause (like an Όταν… clause), just as in English: When …, I ….
You may also see sentences without the comma in informal writing, but the comma is standard and clear.

Why is είμαι used here, and what form is it?

είμαι is the 1st person singular present of είμαι = I am.
Greek often drops the subject pronoun (εγώ) because the verb ending already shows the person, so (εγώ) είμαι = I am.

What does μόνος mean here, and why is it μόνος (not some other form)?

μόνος means alone. It agrees with the (implied) subject I in gender and number.

If the speaker is male: μόνος
If the speaker is female: μόνη
Plural: μόνοι / μόνες (we/they are alone)

So the form tells you something about the speaker’s gender.

Why is it στο σπίτι? What is στο?

στο is a contraction of σε + το:

σε = in/at/to
το = the (neuter)
So στο σπίτι literally means in the house/home.

Why does Greek say στο σπίτι (with the) instead of just at home?

Greek typically uses the definite article where English often doesn’t. στο σπίτι is the normal way to say at home / in the house.
You can also specify my home as στο σπίτι μου.

What case is σπίτι in after στο?

After σε (and therefore after στο), the noun is in the accusative case.
So it’s σε + accusative: στο σπίτι, στη δουλειά, στο σχολείο, etc.

Why is it τη νύχτα and not την νύχτα?

The feminine accusative singular article can be τη(ν). The final -ν is optional in many contexts and is often dropped before most consonants.
Since νύχτα starts with ν (a consonant), τη νύχτα is very common and natural. You may also see την νύχτα in some styles or speakers, but τη νύχτα is standard.

What grammatical role does τη νύχτα have? Is it a direct object?

Here τη νύχτα means at night / during the night. It’s an accusative of time (a very common Greek structure): accusative + article + time noun to express when something happens. It is not the direct object; the direct object is την πόρτα.

Why is the verb κλειδώνω in the present tense if the sentence is about a general habit?

Greek uses the present tense for habitual/general actions, just like English:
When I’m alone at night, I lock the door.
So κλειδώνω here means I (habitually) lock / I lock (in that situation).

Why is it την πόρτα? What case is that, and why the article?

την πόρτα is feminine accusative singular and is the direct object of κλειδώνω (I lock the door).
Greek often uses the definite article for something understood as specific in context (e.g., the door of your home). You could also say κλειδώνω μια πόρτα = I lock a door (more general/less specific).

Could the word order change, or is this the only correct order?

Greek word order is flexible because cases mark roles. This sentence is neutral and natural, but you could also say, for emphasis:

Κλειδώνω την πόρτα όταν είμαι μόνος στο σπίτι τη νύχτα. (focus more on the action)
The meaning stays essentially the same, but the emphasis and flow can shift.