Learn Hardcore Finnish: Torilla on paljon ihmisiä tänään. - There are many people at the market square today.

Torilla (“at the market square”) is the place where the situation happens. That is given as the starting point.
on (“is/are”) comes next.
paljon ihmisiä (“many people”) is the new information being stated about that place.

This structure “place + on + something” is very typical for Finnish existential sentences (“there is / there are”-type sentences).

So Torilla on paljon ihmisiä literally feels more like:
“At the market square there are many people.”

What does torilla literally mean, and what is the -lla ending?

The basic word is tori = “market square, marketplace”.

Torilla = tori + -lla

-lla / -llä is the adessive case.
It usually means “on, at, by” a surface or place.

So torilla literally means “on/at the (market) square”.

Examples with -lla/-llä:

pöydällä = on the table
asemalla = at the station
rannalla = on the shore / at the beach

Why is it torilla and not something like torissa or torille?

Finnish has several locative cases (“where / to / from” cases). Here are three common ones with tori:

torilla (adessive): “on/at the square” → Torilla on paljon ihmisiä.
torille (allative): “to the square” → Menen torille. (“I’m going to the square.”)
torissa (inessive): “in the square / inside the market hall” (used more for enclosed spaces).

In your sentence we are talking about the location where the people are gathered, thought of as an open place you’re “at” or “on”, so torilla is the natural choice.

Why is ihmisiä used here, not ihmiset?

ihmiset = “the people” (nominative plural, a definite group)
ihmisiä = “(some) people” (partitive plural, an indefinite amount)

In Torilla on paljon ihmisiä, we are not talking about some specific, previously known group (“the people we mentioned earlier”). We just mean “many people” in general, an indefinite crowd.

Finnish prefers partitive plural (ihmisiä) in this kind of “there are many / a lot of X” sentence.

What case is ihmisiä, and when do you use it?

Ihmisiä is partitive plural of ihminen (“person, human”).

Singular: ihminen → ihmistä (partitive singular)
Plural: ihmiset → ihmisiä (partitive plural)

You use partitive plural in several situations, including:

Indefinite amounts or quantities
- paljon ihmisiä = a lot of people
- muutamia kirjoja = a few books
After many quantifiers and measure words
- monta ihmistä = many people
- kaksi taloa = two houses (here: number + partitive singular)
In existential sentences when the existence of an indefinite group is stated:
- Kadulla on autoja. = There are (some) cars on the street.
- Torilla on paljon ihmisiä. = There are many people at the market square.

Why is it paljon ihmisiä and not something like paljon ihmisia or paljo ihmisiä?

There are three points here:

Spelling:
- Correct: ihmisiä (with -iä)
- Wrong: ihmisia (missing the dots).
  The -iä ending is partitive plural and follows vowel harmony.
Form of “paljon”:
- paljon is the standard written form meaning “a lot (of), much, many”.
- paljo is a common spoken or colloquial variant, often heard in speech:
  - Siellä on paljo ihmisiä. (spoken)
    In writing and in standard language, paljon is preferred.
Structure:
- paljon + partitive is the normal pattern:
  - paljon ihmisiä = a lot of people
  - paljon vettä = a lot of water

Could I say “monet ihmiset ovat torilla tänään” instead? Does it mean the same?

You can say Monet ihmiset ovat torilla tänään, but the nuance changes:

Torilla on paljon ihmisiä tänään.
- More neutral “there are many people at the square today.”
- Focus on the place (torilla) and the amount of people that happen to be there.
Monet ihmiset ovat torilla tänään.
- More like “many people are at the square today.”
- Focus on the people (many of them) and the fact that they are at the square.

Grammatically both are fine. The first is a typical existential sentence; the second is a more ordinary subject–verb sentence.

Where can I put tänään? Are other word orders possible?

Yes, tänään (“today”) can move around, with small changes in emphasis:

Torilla on paljon ihmisiä tänään. (neutral, very natural)
Tänään torilla on paljon ihmisiä. (emphasis on today)
Torilla tänään on paljon ihmisiä. (slight emphasis on “at the square today”)

What you cannot normally do is split paljon ihmisiä:

✗ Torilla on paljon tänään ihmisiä. (feels wrong)

All the natural versions keep paljon ihmisiä together and place tänään at the start, before on, or at the end.

How would I say “There are not many people at the market square today”?

You negate the verb olla with ei and usually keep the partitive:

Torilla ei ole paljon ihmisiä tänään.
= There are not many people at the market square today.

You can also use paljoa (the partitive of paljon) in negative sentences, which is very common:

Torilla ei ole paljoa ihmisiä tänään.

Both are understood, but paljoa sounds especially natural with ei ole.

How do I turn this into a yes–no question: “Are there many people at the market square today?”

Finnish yes–no questions are usually formed by adding the -ko/-kö question clitic to some word and using rising intonation.

Two natural options:

Attach -ko to on:
- Onko torilla paljon ihmisiä tänään?
  Literally: “Is there at the market square many people today?”
Attach -ko to tänään (stronger emphasis on today):
- Tänäänkö torilla on paljon ihmisiä?
  More like: “(So) today there are many people at the square?”

The first one is the standard neutral question.

Is Torilla on paljon ihmisiä tänään an example of a specific Finnish structure?

Yes. This is a classic existential sentence in Finnish.

General pattern:

[Location / context] + on/oli/… + [NP in partitive]

Examples:

Pihalla on koiria. = There are dogs in the yard.
Kadulla oli paljon autoja. = There were many cars on the street.
Suomessa on paljon järviä. = There are many lakes in Finland.

In these sentences:

The location or context comes first (pihalla, kadulla, Suomessa, torilla).
The verb olla (on / oli / oli ollut, etc.) appears next.
The thing that exists in that place is often in partitive (especially when it’s indefinite or a quantity): koiria, autoja, järviä, ihmisiä.

Your sentence perfectly fits this pattern:

Torilla (location)
on (3rd person singular of olla)
paljon ihmisiä (an indefinite amount of people, partitive plural)
tänään (time adverbial)