Learn Hardcore Finnish: Tämä kahvi on tummempi kuin edellinen. - This coffee is darker than the previous one.

Tämä – demonstrative pronoun in the nominative singular: this. Here it works like a determiner: this coffee.
kahvi – noun in the nominative singular: coffee. It is the subject of the sentence.
on – 3rd person singular of olla: is. It links the subject to the describing adjective (tummempi).
tummempi – comparative form of tumma (dark): darker. It is a predicate adjective describing kahvi.
kuin – conjunction meaning than / as in comparisons.
edellinen – adjective previous / preceding, here used on its own as the previous one. It stands for edellinen kahvi (the previous coffee) with kahvi left out because it is understood from context.

Why is it Tämä kahvi, not Tämän kahvin at the beginning?

The subject of the sentence has to be in the nominative case, so:

Tämä kahvi – both words are nominative; together they form the subject this coffee.

You use tämän kahvin (both in the genitive) only when that whole phrase is not the subject but, for example, a possessor or object:

Tämän kahvin maku on vahva. – The taste of this coffee is strong.
- tämän kahvin = of this coffee (genitive)

In Tämä kahvi on tummempi kuin edellinen, we are simply saying this coffee (subject) is darker than the previous (one), so nominative tämä kahvi is correct.

How is tummempi formed from tumma, and is this the regular way to say darker?

Yes, tummempi is the normal comparative form of tumma (dark).

Formation:

Base adjective: tumma (dark)
Comparative suffix: -mpi

With many adjectives ending in -a / -ä, the vowel in the stem often becomes -e- in the comparative:

tumma → tumme-
- -mpi → tummempi (darker)
Another example: paha (bad) → pahempi (worse)

So:

tumma – dark
tummempi – darker
(superlative for reference: tummin – the darkest)

Why is it tummempi and not tummempaa here?

Both forms exist, but they are used in different situations.

In this sentence:

Tämä kahvi on tummempi kuin edellinen.

tummempi is in the nominative singular because it is a predicate adjective agreeing with a singular, countable subject (tämä kahvi). We are talking about one specific coffee (for example, one cup or one batch) and describing its quality.

You would use the partitive tummempaa when the subject itself is in the partitive or when you are talking about an indefinite amount or a mass:

Kahvi on tummempaa kuin ennen. – Coffee is darker than before.
(Coffee as a substance in general, not one specific “unit” of coffee.)

So:

Specific, countable subject in nominative → tummempi
Indefinite/mass or partitive subject → typically tummempaa

What is the role of kuin, and how is it different from kun?

kuin and kun look and sound similar, but they are different words.

kuin is used in comparisons:
- After comparative adjectives/adverbs:
  - tummempi kuin edellinen – darker than the previous one
- Also in expressions like niin … kuin (as … as).
kun is a conjunction meaning mainly when, as, or because in time/condition clauses:
- Kun tulin kotiin, join kahvia. – When I came home, I drank coffee.

In Tämä kahvi on tummempi kuin edellinen, you must use kuin because you are making a comparison (darker than …). Kun would be ungrammatical here.

Why is edellinen in the basic form with no visible case ending, and where is the word kahvi?

Edellinen is in the nominative singular and is acting like a noun, even though it is originally an adjective.

What is happening:

Full, explicit version could be:
- Tämä kahvi on tummempi kuin edellinen kahvi. – … than the previous coffee.
Finnish often omits a repeated noun when it is obvious from context.
So edellinen kahvi becomes just edellinen, understood as the previous one.

The case of edellinen (nominative) matches the implied kahvi it stands for. If you changed the case because of the grammar, it would change on edellinen too, for example:

Pidän tästä kahvista enemmän kuin edellisestä. – I like this coffee more than the previous one.
- Here tästä kahvista and edellisestä are both in the elative case.

Can you say Tämä kahvi on edellistä tummempi instead of … tummempi kuin edellinen? Is there any difference?

Yes, Tämä kahvi on edellistä tummempi is grammatically correct and natural.

Structure:

Tämä kahvi – subject, nominative
on – verb
edellistä – standard of comparison in the partitive
tummempi – comparative adjective in the nominative

Meaning-wise, both

Tämä kahvi on tummempi kuin edellinen.
Tämä kahvi on edellistä tummempi.

mean essentially This coffee is darker than the previous (one).

Differences are small and mostly stylistic:

… tummempi kuin edellinen
- Very common, clearly comparative thanks to kuin.
… edellistä tummempi
- Slightly more compact; often feels a bit more written or formal, but is also used in speech.

You cannot mix the two patterns into something like tummempi kuin edellistä; either:

tummempi kuin edellinen (with kuin
- nominative), or
edellistä tummempi (no kuin, comparison word in partitive).

Could you also say Tämä kahvi on tummempi kuin se edellinen or … kuin edellinen kahvi? Is that different?

Yes, both are fine:

Tämä kahvi on tummempi kuin se edellinen.
– This coffee is darker than that previous one.
Tämä kahvi on tummempi kuin edellinen kahvi.
– This coffee is darker than the previous coffee.

Compared to the original … kuin edellinen:

kuin edellinen – shortest; edellinen works like the previous one, and kahvi is understood.
kuin se edellinen – adds se (that), which often refers to something already mentioned or slightly more distant; it can make the contrast a bit more explicit.
kuin edellinen kahvi – repeats the noun, so it is very clear you are talking specifically about coffee; stylistically a bit more explicit and possibly more formal.

All three are correct; the original form is just the most economical.

Is the word order fixed? Could I say Tummempi kuin edellinen on tämä kahvi?

The neutral, most common order is:

Tämä kahvi on tummempi kuin edellinen.

Finnish word order is relatively flexible, so you can say:

Tummempi kuin edellinen on tämä kahvi.

This puts strong emphasis on tummempi kuin edellinen (the contrast in color), for example if you are correcting someone or contrasting two coffees in a very focused way.

However:

In normal, everyday speech and writing, you should prefer Tämä kahvi on tummempi kuin edellinen.
You generally cannot omit on in such sentences; Tämä kahvi tummempi kuin edellinen is not standard Finnish. The verb olla (on) is needed to link the subject to the predicate adjective.