Learn Hardcore Greek: Ο ξαφνικός θόρυβος από τον δρόμο ξύπνησε την ανιψιά μου μέσα στη νύχτα. - The sudden noise from the street woke my niece up in the middle of the night.

In Greek, adjectives agree with the noun in gender, number, and case.
Since θόρυβος is masculine singular nominative (the usual case for the subject), the adjective ξαφνικός also appears in masculine singular nominative, which ends in -ος.

So ξαφνικός and θόρυβος both end in -ος because they share:

the same gender (masculine),
the same number (singular),
the same case (nominative).

Why is it Ο ξαφνικός θόρυβος and not Ένας ξαφνικός θόρυβος? Could I use ένας here?

You could say Ένας ξαφνικός θόρυβος από τον δρόμο ξύπνησε την ανιψιά μου...
That would mean “A sudden noise from the street woke my niece up...” — introducing the noise as something not previously known.

Using Ο ξαφνικός θόρυβος suggests:

a more specific noise (for example, one you already mentioned, or that is obvious from context: “the sudden noise (we heard)”),
or a more definite, concrete event in the story.

In English you can say both “A sudden noise” and “The sudden noise” depending on context. Greek makes the same distinction using ένας/μια/ένα (indefinite) vs ο/η/το (definite).

So:

Ο ξαφνικός θόρυβος = the sudden noise
Ένας ξαφνικός θόρυβος = a sudden noise

What does από mean in από τον δρόμο, and why is it used here?

από is a preposition that can mean:

from (origin, source):
- από το σπίτι – from the house
out of, away from (movement):
- φεύγω από την πόλη – I leave the city
because of / due to / caused by (cause):
- κουράστηκα από τη δουλειά – I got tired from/because of work

In από τον δρόμο, it mainly expresses the source of the noise:
“the sudden noise from the street/road”.

So here από shows where the noise comes from.

Why is it τον δρόμο (accusative) after από and not something like ο δρόμος?

In Modern Greek, από (like most prepositions) is normally followed by the accusative case.

ο δρόμος – nominative (subject form)
τον δρόμο – accusative (object / after prepositions)

Because δρόμος follows the preposition από, it must be in the accusative:

από + τον δρόμο

This is a general rule:

από το σπίτι
για την πόλη
με τον φίλο μου All those nouns are in the accusative case.

What is the tense and meaning of ξύπνησε, and how is it different from other forms like ξυπνάει or ξυπνούσε?

ξύπνησε is:

3rd person singular
aorist tense
active voice
of the verb ξυπνάω / ξυπνώ – to wake (up)

Here, it’s used transitively (with a direct object):
Ο ξαφνικός θόρυβος ... ξύπνησε την ανιψιά μου
= “The sudden noise woke my niece up.”

Comparison with other forms:

ξυπνάει / ξυπνά – present tense, “wakes / is waking”
- Ο θόρυβος ξυπνά την ανιψιά μου κάθε μέρα.
  The noise wakes my niece (habitually).
ξυπνούσε – imperfect tense, “was waking / used to wake”
- Ο θόρυβος ξυπνούσε την ανιψιά μου κάθε βράδυ.
  The noise used to wake my niece every night.
ξύπνησε – aorist, a completed event in the past
- Ο θόρυβος ξύπνησε την ανιψιά μου.
  The noise woke her up (once, at that moment).

So ξύπνησε presents the waking as a single, completed action in the past.

Who is the subject and who is the object in this sentence? How can I tell?

The structure is:

Ο ξαφνικός θόρυβος από τον δρόμο – subject (the doer)
ξύπνησε – verb
την ανιψιά μου – direct object (the one affected by the action)

You can recognize this because:

The subject is in the nominative case:
Ο ξαφνικός θόρυβος
The object is in the accusative case:
την ανιψιά μου

Meaning-wise, the noise does the action, and the niece undergoes it.
So: The noise woke (someone) → that someone (my niece) is the object.

Why is it την ανιψιά μου and not just ανιψιά μου? Do you always need the article with a possessive?

In Greek, when you say “my X”, you usually include the definite article:

η μητέρα μου – my mother
ο φίλος μου – my friend
το σπίτι μου – my house
η ανιψιά μου – my niece

Here, because ανιψιά is the direct object, the article appears in the accusative:

την ανιψιά μου – my niece (as an object)

Leaving out the article (ανιψιά μου) is possible in some vocative or very informal contexts (like calling someone: “Έλα εδώ, ανιψιά μου!” – “Come here, my niece!”), but in a normal sentence the article is standard.

So την ανιψιά μου is the normal, grammatically complete form for “my niece” as an object.

Why is μου (my) placed after ανιψιά? Could it go before the noun?

The weak possessive pronoun μου almost always comes after the noun it modifies:

η ανιψιά μου – my niece
ο πατέρας σου – your father
το βιβλίο του – his book

Putting it before the noun (μου η ανιψιά) would sound unnatural or archaic in Modern Greek in this kind of sentence.

So the normal pattern is:

article + noun + possessive pronoun
η ανιψιά μου, τον φίλο μου, το σπίτι μου, etc.

What is the nuance of μέσα στη νύχτα? How is it different from just τη νύχτα?

τη νύχτα – at night / during the night, a general time reference.
μέσα στη νύχτα literally “inside the night”, and idiomatically means:
- “in the middle of the night”
- “during the night (emphasizing it was while the night was ongoing)”

So μέσα στη νύχτα often suggests:

a sense of interiority / being deep into the night,
sometimes closer to English “in the middle of the night” rather than just “at night”.

That’s why in natural English we often translate this as:

“...woke my niece up in the middle of the night.”

Why is it στη νύχτα and not σε τη νύχτα? What exactly is στη?

στη is a contraction of:

σε (preposition: in, at, on, to)
την (feminine accusative singular article: the)

So:

σε + την νύχτα → στη(ν) νύχτα

Greek very often contracts σε + article:

σε + τον → στον (στον δρόμο – in/on the road)
σε + την → στη(ν) (στη νύχτα – in the night)
σε + το → στο (στο σπίτι – in/at the house)
σε + τις → στις (στις πόλεις – in the cities)

So μέσα στη νύχτα = μέσα σε την νύχτα, but the contracted form στη is the normal one.

Why is νύχτα in the accusative case after στη?

The preposition σε (which is hidden in the contraction στη) normally takes the accusative case.

σε + τη νύχτα → στη νύχτα (νύχτα = accusative)
σε + τον δρόμο → στον δρόμο
σε + το σπίτι → στο σπίτι

So νύχτα appears in the accusative because it follows σε (via στη), and that’s simply how Greek prepositions like σε, από, για, etc., work.

Could I change the word order, for example: Η ανιψιά μου ξύπνησε από τον ξαφνικό θόρυβο από τον δρόμο? Does the meaning change?

Yes, you can change the word order, and your sentence is grammatical:

Η ανιψιά μου ξύπνησε από τον ξαφνικό θόρυβο από τον δρόμο.

Differences:

Original:
Ο ξαφνικός θόρυβος από τον δρόμο ξύπνησε την ανιψιά μου...
- Emphasis is more on the noise as the agent (what caused the event).
- Structure: The noise woke my niece.
Alternative:
Η ανιψιά μου ξύπνησε από τον ξαφνικό θόρυβο από τον δρόμο.
- Emphasis is more on the niece and the fact that she woke up.
- The noise is presented as the cause, introduced with από (“from/because of”).

Both mean essentially the same thing:

The niece woke up, and the cause was the sudden noise from the street.

Greek word order is relatively flexible, and you can move elements around to change focus or emphasis, as long as the cases (articles/endings) are correct.