Learn Hardcore Finnish: Viikonlopun elokuva oli todella jännittävä, ja olin koko ajan hereillä. - The weekend’s movie was really exciting, and I was awake the whole time.

Viikonlopun is the genitive form of viikonloppu (weekend). In this sentence it means “of the weekend” or “the weekend’s” and works like an attributive genitive:

viikonlopun elokuva = the weekend’s movie / the movie of the weekend

So viikonlopun is showing a relationship, just like “the weekend’s movie” in English. Using bare viikonloppu elokuva would be ungrammatical; an attributive noun in Finnish is usually put in the genitive (or compounded) when it modifies another noun.

Could this also be viikonloppuelokuva as one word, and if so, what’s the difference from viikonlopun elokuva?

Yes, you could also say:

viikonloppuelokuva = weekend movie (a compound word)
viikonlopun elokuva = the weekend’s movie / the movie for the weekend

The nuance:

viikonloppuelokuva sounds like a type or category, e.g. “a movie that’s (typically) for weekends”.
viikonlopun elokuva is more like a specific movie associated with this particular weekend (for example, “the movie that was on TV this weekend” or “the movie we watched this weekend”).

Both are correct, but the original sentence is talking about a specific, context-known movie.

If it means “the weekend’s movie”, why is there no word for “the” before elokuva?

Finnish has no articles (no “a/an” and no “the”). Definiteness and indefiniteness are usually understood from:

Context
Word order
Whether something is already known / mentioned

So:

elokuva can mean a movie or the movie, depending on context.

In this sentence, because it’s viikonlopun elokuva (the specific movie associated with the weekend), an English speaker would naturally translate it as “the movie”, even though Finnish just uses elokuva with no article.

What exactly does todella mean, and how is it different from words like tosi or erittäin?

Todella is an adverb meaning “really”, “truly”, or “very”. Here:

todella jännittävä = really exciting

Common similar intensifiers:

tosi jännittävä – very colloquial, like “really/so exciting”.
todella jännittävä – neutral, works in both speech and writing.
erittäin jännittävä – a bit more formal: “extremely/very exciting”.
hyvin jännittävä – literally “well exciting”, but idiomatically “very quite exciting” (slightly formal/bookish depending on context).

In casual speech, tosi and todella are very frequent. Erittäin can sound stronger or more formal.

What kind of word is jännittävä grammatically, and how is it formed?

Jännittävä is a present active participle used as an adjective.

It comes from the verb jännittää (to excite, to make tense/nervous):

jännittää → jännittävä = “exciting” (literally “one that excites”)

So grammatically, it’s:

Verb stem jännittä-
- participle ending -vä → jännittävä
Used like a regular adjective: todella jännittävä elokuva = “a really exciting movie”

Why is there a comma before ja (“and”) in …, ja olin koko ajan hereillä? In English that comma would often be left out.

Finnish comma rules are stricter, and you must usually separate two independent clauses with a comma, even when they’re joined by ja:

Viikonlopun elokuva oli todella jännittävä, ja olin koko ajan hereillä.
- Clause 1: Viikonlopun elokuva oli todella jännittävä.
- Clause 2: (Minä) olin koko ajan hereillä.

Both are full clauses with their own verb (oli, olin), so a comma is required before ja.

In English this is optional (and many style guides prefer no comma), but in Finnish omitting the comma here would be considered incorrect or at least non-standard.

What does koko ajan literally mean, and how is it used?

Koko ajan literally means “the whole time”:

koko = whole, entire
aika = time → ajan = genitive form, “of time”

So koko ajan = “(during) the whole time / all the time”.

You can use it similarly in many contexts:

Olin koko ajan hereillä. – I was awake the whole time.
Hän puhui koko ajan. – He/She was talking the whole time.
Satoi melkein koko ajan. – It rained almost the whole time.

Don’t confuse it with:

aina = “always” (in general, habitually), not “the whole time of this one event”.

What is hereillä exactly? It doesn’t look like a basic adjective. Why not just a simple word meaning “awake”?

Hereillä is a special adverb-like form (adessive case) used in the fixed expression olla hereillä = “to be awake”.

Base word: heräillä / hereillä is related to being awake / aroused from sleep.
Form: -llä ending (adessive case), common in many idiomatic “state” expressions.

Examples:

Olen hereillä. – I am awake.
Olin koko yön hereillä. – I was awake all night.
Negative: En ole vielä hereillä. – I’m not awake yet.

You don’t normally replace it with a simple adjective like “awake”; olla hereillä is the natural way to say it.

Is the word order olin koko ajan hereillä fixed, or can you move words around like in English?

The word order is fairly flexible in Finnish, and several variants are natural:

Olin koko ajan hereillä. – neutral, common.
Olin hereillä koko ajan. – also natural; slight emphasis at the end.
Koko ajan olin hereillä. – emphasizes “the whole time” more strongly.

All of these can be used in normal speech, with small differences in emphasis. The original Olin koko ajan hereillä is the most neutral, straightforward order.