Learn Hardcore Finnish: Turnauksen aikana katsomo on melkein täynnä vanhempia ja lapsia. - During the tournament the stands are almost full of parents and children.

[GENITIVE] + aikana = during [something]
- kesän aikana = during the summer
- loman aikana = during the holiday
- turnauksen aikana = during the tournament

So turnaus aikana would be ungrammatical; you must say turnauksen aikana.

What exactly is aikana? Is it a postposition or the word for "time"?

Historically, aikana is the noun aika (time) in the essive case (aika → aikana = as/at the time).

In modern Finnish, in expressions like jonkin aikana it behaves like a postposition meaning during:

sodan aikana = during the war
kurssin aikana = during the course

So in turnauksen aikana, you can think of aikana as “during,” and it always takes a genitive before it.

Could Turnauksen aikana go somewhere else in the sentence?

Yes. Finnish word order is quite flexible. All of these are grammatically correct:

Turnauksen aikana katsomo on melkein täynnä vanhempia ja lapsia.
(Neutral; sets the time frame first.)
Katsomo on turnauksen aikana melkein täynnä vanhempia ja lapsia.
(Slightly more focus on the stands first, then when this happens.)
Katsomo on melkein täynnä vanhempia ja lapsia turnauksen aikana.
(Now the time phrase comes last; it can sound like an added clarification.)

The basic meaning (during the tournament, the stands are almost full) stays the same; word order mainly affects emphasis and information flow.

What does katsomo mean, and why is it in the basic form instead of katsomossa?

Katsomo means “the stands / seating area for spectators” (in a stadium, sports hall, theatre, etc.).

Here:

katsomo is the subject of the sentence.
It appears in the nominative (basic form), as subjects normally do.

So the structure is:

katsomo (subject)
on melkein täynnä (is almost full)
vanhempia ja lapsia (of parents and children)

You could make a different kind of sentence:

Turnauksen aikana katsomossa on paljon vanhempia ja lapsia.
During the tournament, there are many parents and children in the stands.

Here katsomossa is an adverbial (in the stands), and vanhempia ja lapsia becomes the grammatical subject in an existential sentence. That’s a different construction from the original.

Why is it täynnä and not täysi?

The normal way to say “to be full (of something)” in Finnish is olla täynnä (jotakin).

täysi = full (adjective)
täynnä = a special form (etymologically related to an old case form), used in this fixed expression

So we say:

Lasi on täynnä vettä. = The glass is full of water.
Katsomo on täynnä ihmisiä. = The stands are full of people.

Using täysi here (katsomo on melkein täysi) is possible but less common and feels more like describing capacity in a general, “almost at 100%” way. Olla täynnä is the standard idiom when you also mention what it is full of.

Why are vanhempia and lapsia in the partitive plural?

Täynnä normally requires its complement in the partitive case, because it describes the contents as an indefinite mass/amount:

Täynnä mitä? → täynnä vettä, täynnä ihmisiä, täynnä tavaraa

Here:

vanhempia = partitive plural of vanhempi (parent)
lapsia = partitive plural of lapsi (child)

So täynnä vanhempia ja lapsia literally means “full of parents and children” in an indefinite, non-counted way – you’re not specifying which exact parents and which exact children, just that there are lots of them.

Could I say vanhemmat ja lapset instead of vanhempia ja lapsia? What’s the difference?

You can say vanhemmat ja lapset, but it changes the nuance:

vanhempia ja lapsia (partitive plural)
- Feels indefinite and mass-like: “parents and children in general”, “lots of them”.
- Fits naturally with täynnä.
vanhemmat ja lapset (nominative plural)
- Refers more to specific, known groups, something like “the parents and the children”.
- With täynnä, it sounds a bit odd unless you change the structure.

For “full of parents and children” the idiomatic form is täynnä vanhempia ja lapsia (partitive). Using the nominative would usually require a different verb or structure.

Does vanhempia always mean “parents”? Could it just mean “older people”?

Vanhempi has two main uses:

Adjective: older
- vanhempi veli = older brother
Noun: a parent
- Olen kahden lapsen vanhempi. = I’m the parent of two children.

In the plural:

vanhemmat = parents (as a noun)
vanhempia (partitive plural) = parents (some parents)

In your sentence, vanhempia clearly means parents, because it is paired with lapsia (children).
If you wanted to say “older people”, you’d typically use words like vanhoja ihmisiä (old people) rather than vanhempia by itself.

What exactly does melkein do here, and where can it go?

Melkein means “almost, nearly”.

In on melkein täynnä, it modifies täynnä:

täynnä = full
melkein täynnä = almost full

Typical positions:

Katsomo on melkein täynnä vanhempia ja lapsia.
(Most natural: almost full of parents and children.)

You could also say:

Katsomo on melkein ihan täynnä. (almost completely full)
Melkein koko katsomo on täynnä. (almost the whole stands are full)

Putting melkein right before on (e.g. melkein katsomo on täynnä) usually sounds wrong or changes the meaning in an odd way. The safe pattern is melkein + adjective/adverb/quantity.

Why is the verb on singular, even though vanhempia ja lapsia refers to many people?

The subject of the sentence is katsomo (singular), not vanhempia ja lapsia.

Structure:

katsomo = subject (singular)
on = verb “is” (3rd person singular)
melkein täynnä vanhempia ja lapsia = predicate describing the state of the stands

So the verb agrees with katsomo, which is singular:

Katsomo on täynnä. = The stands is/are full.
Katsomot ovat täynnä. = The stands (plural) are full.

The phrase vanhempia ja lapsia is part of the complement of täynnä, not the subject, so it doesn’t control the verb form.