Learn Hardcore Finnish: Minun vatsani on kipeä tänään.

vatsani alone already means my stomach and is fully correct.
minun vatsa (without -ni) is very common in colloquial speech, but is not “best” standard written Finnish.

So:

Standard: Vatsani on kipeä tänään. / Minun vatsani on kipeä tänään.
Colloquial: Mun vatsa on kipeä tänään.

What grammatical case is vatsani in?

Vatsani is in the nominative singular.

Base form: vatsa (stomach) – nominative
Add possessive suffix -ni (my) → vatsani

The possessive suffix does not change the case; it just attaches to the noun in whatever case the noun already has.

Examples of other cases with the same suffix:

vatsassani = in my stomach (inessive: vatsassa
- -ni)
vatsaani = to my stomach / my stomach (partitive or illative, depending on context)

Is it necessary to use both minun and -ni, or is that redundant?

Strictly speaking, it is redundant in meaning: both mark “my”. However:

In traditional/standard written Finnish, using both (especially with emphasis) is normal and correct:
- Minun vatsani on kipeä tänään. (slight emphasis on my stomach)
Using just the possessive suffix is also completely correct and somewhat more compact:
- Vatsani on kipeä tänään.
In everyday spoken Finnish, people often use the pronoun and drop the suffix:
- Mun vatsa on kipeä tänään.

So:

Both minun vatsani and vatsani are correct standard.
Minun vatsa is colloquial (common in speech, less appropriate in formal writing).

Could I simply say Vatsa on kipeä tänään without any “my”? Would that still mean my stomach?

In a real conversation, Vatsa on kipeä tänään will almost always be interpreted as my stomach is sore today, because the speaker is obviously talking about their own body.

Finnish often omits explicit “my/your” with body parts when it’s clear whose body part is meant. So:

Vatsa on kipeä. – My stomach hurts.
Pää on kipeä. – My head hurts.

If the context were unusual (e.g. a doctor talking about an X‑ray of someone else), then you’d clarify whose stomach you mean, but in ordinary everyday conversation, no extra marking is needed for “my” here.

Why do we use on kipeä (“is sore”) instead of a specific verb like “hurts”, for example sattuu?

Finnish has two common patterns here:

“is sore/ill” with the adjective kipeä:
- Vatsani on kipeä. – My stomach is sore.
- Focus on the state: the stomach is in a painful/sore condition.
“hurts” with the verb sattua / särkeä:
- Vatsaan sattuu. – My / someone’s stomach hurts.
- Vatsani särkee. – My stomach aches.
- Focus more directly on the sensation of pain.

Both are natural. On kipeä sounds a bit more neutral and descriptive; sattuu/särkee can sound more directly like you’re complaining of pain.

What’s the difference between kipeä and sairas?

Both relate to illness or not being well, but they’re used differently:

kipeä
- Often used for body parts that hurt:
  - Vatsani on kipeä. – My stomach is sore.
  - Jalkani on kipeä. – My leg is sore.
- Can also mean emotionally painful or “sensitive” in some contexts.
sairas
- Means ill, sick in a more general sense (the person is sick):
  - Olen sairas. – I am sick.
  - Olen tänään sairaslomalla. – I’m on sick leave today.

So Minun vatsani on kipeä tänään focuses specifically on pain in the stomach, not that you as a whole person are generally “sick”.

Can I move tänään to another place in the sentence, like Tänään minun vatsani on kipeä or Minun vatsani on tänään kipeä?

Yes. Finnish word order is fairly flexible, and tänään (today) can be placed in several positions:

Minun vatsani on kipeä tänään.
Minun vatsani on tänään kipeä.
Tänään minun vatsani on kipeä.
Tänään vatsani on kipeä.

All are grammatically correct. Differences are mostly about emphasis:

Starting with tänään emphasizes “today (as opposed to another day)”.
Moving tänään next to kipeä can slightly emphasize the time of the state: it’s sore today.

For a neutral statement, the original version or Minun vatsani on tänään kipeä are very natural.

Is the verb on (is) always required here, or can Finnish drop it like in some headlines (“Stomach sore today”)?

In normal full sentences, the verb olla (“to be”) is required:

Vatsani on kipeä tänään. – correct
Vatsani kipeä tänään. – feels like a headline, note, or fragment, not a full sentence.

You might see the verb omitted in:

Headlines
Lists, notes, labels
Very telegraphic or poetic style

But for ordinary spoken or written Finnish, keep on in this kind of sentence.

How is vatsani pronounced exactly? What about kipeä and tänään?

Key points:

Stress is always on the first syllable in Finnish.

vatsani

Syllables: vat-sa-ni
ts like in English “cats”.
a like “a” in “father” (open a).
Roughly: VAHT-sah-nee (all vowels clear and short).

kipeä

Syllables: ki-pe-ä
i like “ee” in “see” but short.
e like “e” in “get”.
ä like “a” in “cat”, but a bit clearer and more fronted.
Roughly: KEE-peh-ae (three separate syllables, final ä not reduced).

tänään

Syllables: tä-nään
ä as above (like “a” in “cat”).
The double ää is a long vowel – hold it longer.
Final n is pronounced clearly.
Roughly: TAE-naa(n) with the second vowel in -nään clearly longer than a short ä.

Is there another common way to express the same idea, like “I have a stomachache” in English?

Yes, Finnish also often uses constructions with minulla on (“I have”) and a noun or pain word:

Minulla on vatsakipu. – I have a stomachache.
Minulla on vatsakipua. – I have some stomach pain.
Minulla on vatsa kipeä. – Literally: “I have the stomach sore.”

Compare:

Minun vatsani on kipeä tänään. – My stomach is sore today.
Minulla on vatsakipua tänään. – I have stomach pain today.

Both are natural; which you choose is mostly a stylistic preference. The original sentence with on kipeä is very common and idiomatic.