Learn Hardcore Finnish: Minä olen varma, että kirja on hyvä.

emphasize you specifically:
Minä olen varma, mutta hän ei ole. – I am sure, but he/she is not.
speak very clearly as a beginner, or in situations where you want to avoid any ambiguity.

So both are correct, but the natural, neutral everyday version is without minä.

What exactly does että do here? Is it the same as English that?

Yes. In this sentence että is a conjunction that introduces a subordinate clause, just like English that:

Minä olen varma, että kirja on hyvä.
I am sure that the book is good.

Structure:

main clause: Minä olen varma
että
subordinate clause: kirja on hyvä

Some English speakers drop that (e.g. I’m sure the book is good), but in Finnish you cannot drop että in this kind of sentence.
✗ Minä olen varma, kirja on hyvä is wrong; you must have että.

Why is there a comma before että?

Finnish normally uses a comma before että when it introduces a subordinate clause.

Minä olen varma, että kirja on hyvä.

This is a punctuation rule, not a pause rule. You might pause there in speech, but the comma is required even if you say it smoothly.

Typical patterns:

Tiedän, että... – I know that…
Luulen, että... – I think that…
Toivon, että... – I hope that…
Olen varma, että... – I am sure that…

Why is it kirja and not kirjan or kirjaa?

Here kirja is the subject of the verb on in the subordinate clause kirja on hyvä:

kirja – nominative singular (dictionary form)
on – 3rd person singular of olla (to be)
hyvä – predicate adjective describing the subject

The basic pattern X is Y is:

Subjekti (nominative)
- on
  - predikatiivi (nominative)
    Kirja on hyvä. – The book is good.

So kirja must be in the nominative. Forms like kirjan (genitive) or kirjaa (partitive) would change the meaning or be ungrammatical in this simple descriptive sentence.

Why is it hyvä and not hyvää or hyvin?

Hyvä is the basic adjective form (nominative). In kirja on hyvä, hyvä is a predicate adjective that describes the subject kirja, and it appears in the nominative:

Kirja on hyvä. – The book is good.

Other similar examples:

Elokuva on pitkä. – The movie is long.
Ruoka on kallista. → here there is a partitive (kallista) for other reasons, but the simple form is kallis.

The alternatives you mentioned:

hyvää – partitive form of hyvä; used in some structures, e.g. Se on hyvää about an unspecified mass (like food or drink), but not here with kirja as a concrete subject.
hyvin – adverb, meaning well. It modifies verbs, not nouns:
Hän puhuu hyvin suomea. – He/she speaks Finnish well.

In our sentence we need an adjective describing kirja, so hyvä is correct.

Is varma an adjective? Does it have to agree with minä in some way?

Yes, varma is an adjective meaning sure, certain.

In Minä olen varma, varma is a predicate adjective describing the subject minä:

Minä olen varma. – I am sure.
Sinä olet varma. – You are sure.
Hän on varma. – He/She is sure.

In Finnish, adjectives do not change for person or gender. So you always use varma here, regardless of whether the subject is minä, sinä, hän, me, te, or he.

They do agree in number and case with the noun they modify, but in this kind of olla + adjective structure, the adjective is in the nominative singular and does not change for person.

Can I say Minä olen varma kirjasta instead of using että?

Olla varma jostakin is a different pattern: to be sure about something.

Minä olen varma kirjasta.
Literally: I am sure about the book.

This sounds a bit odd without more context. You would more naturally say:

Olen varma siitä, että kirja on hyvä.
I am sure that the book is good.
(Literally: I am sure about it, that the book is good.)

Patterns:

Olla varma, että…
- full clause
- Olen varma, että kirja on hyvä.
Olla varma jostakin (elative: -sta/-stä)
- Olen varma siitä, että…
- Olen varma päätöksestäni. – I am sure about my decision.

So you can use kirjasta with varma, but it changes the structure and the nuance. The most natural equivalent of English I am sure that… is Olen varma, että…

Is the word order fixed, or can I change it, like Varma olen, että kirja on hyvä?

The normal, neutral word order is:

(Minä) olen varma, että kirja on hyvä.

Finnish word order is more flexible than English, but changes often add emphasis or sound poetic/archaic. For example:

Varma olen, että kirja on hyvä. – Emphasizes varma; sounds poetic or very expressive.
Kirja on hyvä, olen siitä varma. – The book is good, I’m sure of that. Different structure.

For everyday speech and writing, stick to:

Olen varma, että kirja on hyvä.

Changing word order is possible, but you should first master the basic pattern before experimenting with emphasis.

Can I omit että, like in English I’m sure the book is good?

No. In Finnish you must keep että in this type of clause.

Compare:

English: I’m sure (that) the book is good.
→ that is optional
Finnish: Olen varma, että kirja on hyvä.
→ että is not optional

The version without että:

✗ Olen varma, kirja on hyvä.

is ungrammatical. Whenever you have a verb like tiedän, luulen, toivon, pelkään, olen varma, etc. followed by a full clause, you should expect to use että.

Shouldn’t there be a future tense for kirja on hyvä if I mean “the book will be good”?

Finnish generally uses the present tense where English might use the future:

Luulen, että kirja on hyvä.
Can mean: I think the book is good or I think the book will be good, depending on context.

There is a future-like construction using tulla (tulee olemaan), but it is used less often and usually for stronger, more explicit future prediction:

Kirja tulee olemaan hyvä. – The book will (definitely) be good. (More emphatic / formal / predictive)

In your sentence, kirja on hyvä is perfectly normal for both present and future meaning, and Olen varma, että kirja on hyvä is the most natural version.

Could I say Olen ihan varma, että kirja on hyvä? What does ihan do?

Yes, and that’s very common and natural.

Olen varma, että kirja on hyvä. – I am sure that the book is good.
Olen ihan varma, että kirja on hyvä. – I am completely / totally sure that the book is good.

Here ihan functions as an intensifier: quite, totally, absolutely. It makes your certainty sound stronger and more colloquial.

Other examples:

Olen ihan varma. – I’m absolutely sure.
Olen melko varma. – I’m fairly sure.

Why doesn’t hyvä change to match kirja? In some languages adjectives change a lot.

In Finnish, adjectives do agree with nouns in number and case, but in this sentence both kirja and hyvä are in the singular nominative, so they just look like their dictionary forms:

kirja – nominative singular
hyvä – nominative singular

If you change the number or case, you see agreement:

Hyvät kirjat ovat kalliita. – Good books are expensive.
(hyvät plural nominative, matching kirjat)
Ostan hyvän kirjan. – I buy a good book.
(hyvän genitive, matching kirjan)

In kirja on hyvä, they are both nominative, so no extra endings are added, and they already match correctly.

How would I say this more casually or in spoken language?

A typical casual spoken version would be:

Mä oon varma, että kirja on hyvä.

Changes compared to the standard written form:

minä → mä (colloquial 1st person singular)
olen → oon (colloquial form of olla)
Comma and että remain the same in writing; in speech the comma just corresponds to a slight pause or none at all.

In very relaxed speech people might even shorten että a bit in pronunciation, but it’s still että in writing.