Learn Hardcore Finnish: Puoliajan aikana katsomo oli täynnä puhetta, musiikkia ja naurua. - During halftime the stands were full of talk, music, and laughter.

You use the genitive because it comes before the postposition aikana (“during”), which requires the preceding noun to be in the genitive case. So literally it’s something like “during the time of halftime”.

What exactly is aikana, and why do we say puoliajan aikana?

Aikana is a postposition meaning “during” or “in the time of”.

Structure:

noun in genitive + aikana
puoliajan aikana = “during halftime”

Other examples:

Loman aikana – during the vacation
Talven aikana – during the winter

So aikana always follows a noun in the genitive and expresses “during that period”.

Could I also say puoliajalla instead of puoliajan aikana? Are both correct?

Yes, both are correct, but they feel slightly different:

puoliajan aikana – literally “during halftime” (more explicitly time-like)
puoliajalla – adessive case of puoliaika, literally “at (the) halftime”

In practice:

Puoliajan aikana katsomo oli täynnä…
Puoliajalla katsomo oli täynnä…

Both are natural. Puoliajan aikana feels a bit more formal or explicit; puoliajalla is somewhat more compact and common in speech.

What does katsomo mean, and is it “stands” or “audience”?

Katsomo means the seating area where people watch something: the stands, bleachers, or auditorium seating. It refers to the place, not directly to the people.

katsomo – the stand(s) / spectator area
katsoja – a spectator (a person who watches)

In English translation you often see “the stands” or “the audience”, depending on context, but the Finnish word itself is location-oriented.

How does oli täynnä work? Is it like “was full of”?

Yes, oli täynnä literally means “was full (of)”.

Structure:

olla (to be) + täynnä (full) + something in the partitive case

Example:

Katsomo oli täynnä ihmisiä. – The stands were full of people.
Lasi on täynnä vettä. – The glass is full of water.

So here:

katsomo oli täynnä puhetta, musiikkia ja naurua
= the stands were full of talk, music, and laughter.

Why are puhetta, musiikkia, naurua in the partitive case, not just puhe, musiikki, nauru?

Because after täynnä, Finnish uses the partitive to show an indefinite amount or a kind of “filling substance”.

puhetta – partitive of puhe (speech/talk)
musiikkia – partitive of musiikki (music)
naurua – partitive of nauru (laughter)

The partitive here implies “full of some (non-countable) amount of X”:

täynnä puhetta – full of (the sound of) talk
täynnä musiikkia – full of music
täynnä naurua – full of laughter

Using nominative (puhe, musiikki, nauru) would be ungrammatical in this structure. The pattern is:
olla täynnä + partitive.

Are puhetta, musiikkia, naurua singular or plural forms?

They are partitive singular forms:

puhe → puhetta (partitive singular)
musiikki → musiikkia (partitive singular)
nauru → naurua (partitive singular)

They behave like mass/uncountable nouns here: we are not counting individual “talks” or “laughters”, but describing a general presence of these things filling the space.

Partitive plural would be:

puheita, musiikkeja, nauruja – these would usually refer to multiple distinct items (e.g. different speeches, different types of music, individual laughs), and would sound odd in this sentence.

What is the difference between puhe, puhetta and the verb puhua?

puhua – to speak, to talk (verb)
- Minä puhun. – I speak.
puhe – speech, talk (noun)
- Hänen puheensa oli hyvä. – His/her speech was good.
puhetta – partitive of puhe
- Täällä on paljon puhetta. – There’s a lot of talk here.

In your sentence, puhetta means “(the sound of) talk / talking”, as a general background noise.

What is the difference between nauru, naurua, and the verb nauraa?

nauraa – to laugh (verb)
- He nauravat. – They are laughing.
nauru – laughter, a laugh (noun)
- Hänen naurunsa on tarttuvaa. – His/her laugh is contagious.
naurua – partitive of nauru
- Paljon naurua. – A lot of laughter.

In the sentence, naurua means “laughter (as a general sound/atmosphere)” filling the stands.

Can I change the word order, for example: Katsomo oli puoliajan aikana täynnä puhetta, musiikkia ja naurua?

Yes, that word order is also grammatically correct:

Puoliajan aikana katsomo oli täynnä…
Katsomo oli puoliajan aikana täynnä…

Both are fine.

Word order in Finnish mainly affects emphasis and flow, not basic grammar:

Starting with Puoliajan aikana emphasizes the time frame (“During halftime, …”).
Starting with Katsomo emphasizes the location (“The stands were, during halftime, full of …”).

The core meaning remains the same.