Learn Hardcore Finnish: Tämä runokirja on lyhyt mutta kaunis. - This poetry book is short but beautiful.

tämä = this (near me)
tuo = that (a bit further away, but visible)
se = it / that (already known from context, not necessarily visible or relevant where it is)

In your sentence, Tämä runokirja is like saying “This poetry book” (the one I’m holding / pointing at / have just shown you).
If you said:

Tuo runokirja on lyhyt mutta kaunis.
→ That poetry book (over there) is short but beautiful.
Se runokirja on lyhyt mutta kaunis.
→ That / The poetry book (we already talked about) is short but beautiful.

The basic grammar stays the same; only the “distance” or discourse status changes.

Why is runokirja written as one word and not runo kirja?

Runokirja is a compound noun: runo (poem) + kirja (book) → runokirja (a book of poems / poetry book).

In Finnish, when two nouns together create a single new concept, they are normally written as one word, unlike English which often uses two words (e.g. snowman, coffee table vs Finnish lumimies, sohvapöytä).

If you wrote runo kirja as two words, it would sound like you are just listing two separate things: a poem, a book, rather than “a poetry book”.

So:

runo
- kirja → runokirja (one specific kind of book)

What case is runokirja in, and why does it look like the basic dictionary form?

Runokirja is in the nominative singular case, which usually looks exactly like the dictionary form.

In this sentence:

Tämä runokirja = the subject
on = verb “to be” (3rd person singular)
lyhyt and kaunis = adjectives describing the subject (predicative)

With the verb olla (to be), when you say X is Y, both X and Y are typically in nominative:

Tämä runokirja (nominative subject)
lyhyt and kaunis (nominative predicatives agreeing with the subject)

You would use a different case (often partitive) if you were expressing something like “This poetry book is somewhat short” in a more nuanced or partial sense, but the basic statement uses nominative.

Why are the adjectives lyhyt and kaunis in that form, and do adjectives have to agree with the noun?

Yes, Finnish adjectives agree with the noun in number (singular/plural) and case.

Here, lyhyt (short) and kaunis (beautiful) describe Tämä runokirja:

The noun runokirja is singular nominative.
The adjectives lyhyt and kaunis are also singular nominative.

So you get:

Tämä runokirja on lyhyt mutta kaunis.

If the noun were plural, the adjectives would also be plural:

Nämä runokirjat ovat lyhyitä mutta kauniita.
(These poetry books are short but beautiful.)
→ Now everything is plural, and because of the plural + partitive predicative, the endings change: -ita / -itä.

So in your original sentence, the adjectives match runokirja in being singular nominative, which is why they appear in their basic forms lyhyt and kaunis.

What is on, and why does it not change according to “this poetry book”?

On is the third person singular form of the verb olla (to be).

The present tense of olla is roughly:

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

Your subject Tämä runokirja is third person singular, so you must use on:

Tämä runokirja on lyhyt mutta kaunis.
This poetry book is short but beautiful.

If you changed the subject to plural:

Nämä runokirjat ovat lyhyitä mutta kauniita.
→ Now runokirjat is plural, so the verb becomes ovat (they are).

Why is it mutta and not ja? What is the difference between mutta and ja here?

mutta means “but”
ja means “and”

So:

lyhyt ja kaunis = short and beautiful
→ simply adds one positive quality to another
lyhyt mutta kaunis = short but beautiful
→ introduces a contrast (in many contexts “short” might be seen as a downside, but you’re saying despite being short, it is beautiful).

You could say:

Tämä runokirja on lyhyt ja kaunis.
→ just lists two neutral/positive properties

But with mutta, you imply a small opposition between the two qualities.

What is the difference between “Tämä runokirja on lyhyt mutta kaunis” and “Tämä lyhyt mutta kaunis runokirja on…”?

In your original sentence:

Tämä runokirja on lyhyt mutta kaunis.
→ You first identify the book, then comment on it:
This poetry book (as a topic) is short but beautiful.
The adjectives are predicative (after the verb), describing the subject.

If you say:

Tämä lyhyt mutta kaunis runokirja on pöydällä.
→ This short but beautiful poetry book is on the table.

Here:

lyhyt mutta kaunis is placed before the noun and is attributive, like English this short but beautiful poetry book.
The main new information in the sentence becomes “on pöydällä” (is on the table).

So:

Adjectives after on → comment describing the subject’s qualities.
Adjectives before the noun → part of the noun phrase itself, identifying which book you mean.

Why doesn’t Finnish use any word like “a” or “the” in this sentence?

Finnish has no articles like English a / an / the.

The noun runokirja can be translated as either:

“a poetry book”
“the poetry book”

depending on context. The demonstrative tämä already makes it definite:

Tämä runokirja → this poetry book → clearly definite.

If you just said:

Runokirja on lyhyt mutta kaunis.
Depending on context, this could be understood as
- The poetry book is short but beautiful.
- or more abstractly A poetry book is short but beautiful.

But the language itself doesn’t mark the difference with an article; it relies on context, word choice (like tämä, se, etc.), and word order.

How do you pronounce Tämä runokirja on lyhyt mutta kaunis?

Pronunciation tips:

Stress is always on the first syllable of each word.
Every written vowel is pronounced, and the spelling is very phonetic.

Approximate breakdown (primary stress in CAPS):

TÄ-mä – “TÆ-ma” (ä like in cat, short)
RU-no-kir-ja – “ROO-no-keer-ya”
- r is trilled
- j is like English y in yes
on – like English “on” but shorter
LY-hyt – “LUH-hyt”; y is a rounded front vowel, like French u in tu
MUT-ta – “MOOT-ta”; tt is a long t (hold it slightly)
KAU-nis – “KOW-nees” (like cow but shorter “ow”)

Very roughly in English-ish sounds:

TÄ-mä ROO-no-keer-ya on LUH-hyt MOOT-ta KOW-nees

But the key points: stress on first syllables, roll the r, pronounce j as y, and keep vowels pure and short/long as written.

Is there any consonant gradation or stem change in the words from this sentence when they are inflected?

Some of these words will change form when inflected, even though they appear in their basic forms here.

Two relevant examples:

kirja (book)
- nominative: kirja
- genitive (of the book): kirjan
- partitive (book, as an object/quantity): kirjaa
In this word, there is no consonant gradation; the consonants stay the same.
lyhyt (short)
- nominative: lyhyt
- genitive: lyhyen
- partitive: lyhyttä
Here you see a stem change: the basic stem is lyhyt- / lyhy-, and in most cases you see lyhy- before the ending: lyhyttä, lyhyen, etc.

So in your sentence, everything is in a simple nominative form (no visible gradation), but when you start declining these words, you will see stem alternations like lyhyt → lyhyttä / lyhyen.