Learn Hardcore Finnish: Tämä kahvila on halvempi kuin ravintola keskustassa. - This café is cheaper than the restaurant in the city centre.

Tämä kahvila = “this café” (you are specifying which café).
If you said just Tämä on kahvila, it would mean “This is a café.” That introduces or identifies it as a café.
In the given sentence, we already know it’s a café; now we’re describing a property of this particular café, so we say Tämä kahvila on… (“This café is …”).

So tämä + noun works like “this + noun” in English: this café, this car, this house → tämä kahvila, tämä auto, tämä talo.

Why is kahvila in its basic dictionary form? Shouldn’t there be some ending?

The basic form kahvila (nominative singular) is used because it is the subject of the sentence.

Subject in singular → usually nominative singular (no ending).
Verb: on (“is”) → 3rd person singular.
Subject matching the verb: (Tämä) kahvila on … (“This café is …”).

You add endings when the word has some other role (location, object, etc.), but here kahvila simply answers “Who/what is cheaper?” → kahvila.

What form of the verb is on, and why not olla?

Olla is the infinitive form: “to be”.

On is the 3rd person singular present tense of olla:

minä olen – I am
sinä olet – you are
hän / se on – he/she/it is
me olemme – we are
te olette – you (pl) are
he / ne ovat – they are

Your sentence needs “is”, so you use on:
Tämä kahvila on … = “This café is …”

Why is it halvempi and not halpampi or something with enemmän (like “enemmän halpa”)?

Halvempi is the comparative form of the adjective halpa (“cheap”).

Finnish usually forms the comparative by adding -mpi to the stem of the adjective, but the stem can change:

halpa → stem halva- → halvempi (“cheaper”)
kallis (“expensive”) → kalliimpi (“more expensive”)
nopea (“fast”) → nopeampi (“faster”)

You don’t say enemmän halpa. The comparative is built into the word halvempi itself, so:

halpa = cheap
halvempi = cheaper
halvin = the cheapest (superlative)

What does kuin do here, and is it always needed in comparisons like this?

Kuin in this sentence is a comparison word, similar to “than” in English.

Pattern:

X on adj-empi kuin Y
= “X is adj-er than Y”

So:

Tämä kahvila on halvempi kuin ravintola keskustassa.
= “This café is cheaper than the restaurant in the city centre.”

In this basic comparative structure, kuin is necessary. You can’t drop it here. Without kuin, the sentence would feel incomplete or change meaning.

Why is ravintola also in the basic form and not something like ravintolaa or ravintolan?

In the structure:

X on halvempi kuin Y

the Y (the thing you compare to) is normally in the nominative (basic form):

Tämä kahvila on halvempi kuin ravintola.
“This café is cheaper than (the) restaurant.”

So ravintola is in the same “dictionary form” as kahvila.

There is another common structure, where the compared item is in the partitive:

Tämä kahvila on ravintolaa halvempi.
Literally “This café is restaurant-PART cheaper.”

Both are correct, but your sentence uses the kuin + nominative pattern, which is probably the simplest for learners.

Why does keskustassa have the ending -ssa? What does that ending mean?

The ending -ssa / -ssä is the inessive case, meaning “in / inside”.

keskusta = city centre, downtown
keskustassa = in the city centre

So:

ravintola keskustassa = “(the) restaurant in the city centre”

Compare other common cases with keskusta:

keskustaan (illative) = to the city centre
keskustasta (elative) = from the city centre
keskustassa (inessive) = in the city centre

Your sentence describes where the restaurant is, so -ssa (“in”) is used.

Could the word order be different, like putting keskustassa earlier in the sentence?

Finnish word order is fairly flexible, so you can move parts around, but some options sound more natural than others.

Your original:

Tämä kahvila on halvempi kuin ravintola keskustassa.

Other natural variants:

Tämä kahvila on halvempi kuin keskustassa oleva ravintola.
(“…than the restaurant that is in the city centre.”)
Tämä kahvila on keskustassa olevaa ravintolaa halvempi.
(uses the partitive comparison; slightly more formal/complex)

Less natural or confusing:

Tämä kahvila on keskustassa halvempi kuin ravintola.
Could be misread as “In the city centre, this café is cheaper than the restaurant,” i.e. it sounds like kahvila is in the centre.

So yes, you can move words, but keep the structure clear: keep ravintola keskustassa together if you want to strongly show that the restaurant (not the café) is in the centre.

How do we know if ravintola means “a restaurant” or “the restaurant”? There’s no “the” or “a” in Finnish.

Finnish has no articles (a/an, the). The noun ravintola on its own can mean:

“a restaurant”
“the restaurant”

Which one is correct depends on context, not on the word form.

In this sentence:

Tämä kahvila on halvempi kuin ravintola keskustassa.

In English, both are possible:

“This café is cheaper than a restaurant in the city centre.”
“This café is cheaper than the restaurant in the city centre.”

If you are talking about a specific, known restaurant, you translate with “the”. If you mean any restaurant in the centre, you could use “a”. The Finnish sentence itself leaves that open.

How would I turn this into a yes/no question in Finnish?

To make a yes/no question, Finnish usually attaches -ko / -kö to a word (often the verb) and inverts the intonation:

Statement: Tämä kahvila on halvempi kuin ravintola keskustassa.
Question: Onko tämä kahvila halvempi kuin ravintola keskustassa?
= “Is this café cheaper than the restaurant in the city centre?”

Rules:

Add -ko after a word ending in a non-front vowel (a, o, u)
Add -kö after a word with a front vowel (y, ä, ö, or i/e in some patterns)
Here: on → onko

So simply: Onko tämä kahvila …? = “Is this café …?”