Learn Hardcore Finnish: Istun illalla pehmeään nojatuoliin lukemaan kirjaa. - In the evening I sit down in a soft armchair to read a book.

nojatuoliin – illative: into the armchair / onto the armchair (movement, goal)
nojatuolissa – inessive: in/on the armchair (static location)

Compare:

Istun nojatuolissa. – I sit (am sitting) in/on the armchair.
Istun nojatuoliin. – I sit down into the armchair (I move to sit there).

In your sentence Istun illalla pehmeään nojatuoliin lukemaan kirjaa, the speaker is describing the action of sitting down into the chair, often understood as a little ritual: in the evening, I settle into the soft armchair to read a book.

If you used nojatuolissa instead:

Istun illalla pehmeässä nojatuolissa lukemassa kirjaa.
→ I (am) sit(ting) in the soft armchair reading a book. (Focus on being there and reading, not on the act of sitting down.)

Isn’t istua a “static” verb? How can it be used with a movement case like nojatuoliin?

In many languages, sit is purely static, but Finnish istua is flexible:

With a static case (inessive: -ssa/-ssä), it means simply to be sitting somewhere:
- Istun nojatuolissa. – I am (sitting) in the armchair.
With a directional case (illative: -in/-hin, etc.), it can mean to sit down (into/on):
- Istun nojatuoliin. – I sit down in/into the armchair.

So the construction istua + illative focuses on the transition into the seated position in that place, not just the state of already sitting there.

What exactly is lukemaan? Why not just lukea?

Lukemaan is the 3rd infinitive in the illative (often called the MA-infinitive illative) of the verb lukea (to read).

Dictionary form: lukea
3rd infinitive stem: lukema-
Illative: lukemaan

This form is widely used with verbs of going, coming, staying, sitting, etc. to express purpose or the beginning of an activity:

mennä lukemaan – to go (in order) to read
tulla lukemaan – to come to read
jäädä lukemaan – to stay to read
istua lukemaan – to sit down to read

So istun … lukemaan ≈ I sit down in order to read.

Using just lukea here (istun lukea) would be grammatically wrong in Finnish; purpose after these verbs is expressed with -maan/-mään, not the basic infinitive.

Why is it lukemaan kirjaa and not lukemaan kirjan?

Both are possible, but they mean different things.

Kirjaa is partitive singular:

lukemaan kirjaa – to read (some of) a book / to spend time reading a book
- Ongoing, incomplete or unbounded activity
- Focus on the activity, not on finishing the whole book

Kirjan is genitive singular:

lukemaan kirjan – to (set out to) read the whole book
- Bounded, complete event
- Implies an intention to finish it

In everyday speech, lukemaan kirjaa is more natural if you just mean you’re going to sit and read, without stressing that you will finish the entire book that evening.

Why is kirjaa in the partitive? Is reading always with the partitive?

Not always, but very often.

The object of lukea can be:

Partitive (kirjaa) → the action is ongoing, incomplete, or unbounded:
- Luen kirjaa. – I’m reading a book (currently; not necessarily finishing it).
- Istun lukemaan kirjaa. – I sit down to read (some of) a book.
Genitive (kirjan) → the action is seen as whole/completed:
- Luen kirjan. – I will read the (whole) book (and finish it).
- Istun lukemaan kirjan. – I sit down intending to read the whole book.

So in the original sentence, the partitive kirjaa matches the idea of a relaxed, ongoing activity in the evening.

Could the sentence just be Istun illalla pehmeään nojatuoliin lukemaan, without kirjaa?

Yes, that is perfectly grammatical:

Istun illalla pehmeään nojatuoliin lukemaan.
→ In the evening I sit down into the soft armchair to read.

Leaving out kirjaa makes the object of reading unspecified – maybe a book, maybe a magazine, maybe something on a tablet; the key idea is “I sit down to read (something).”

Adding kirjaa simply specifies what you read and gives a slightly more concrete, cozy image.

What’s the difference between lukemaan and lukemassa after istua?

Both are forms of the 3rd infinitive, but they express different aspects:

lukemaan – illative (-maan/-mään)
- Focus on the start/purpose of the activity
- “to sit down in order to read”
→ Istun nojatuoliin lukemaan kirjaa.
I sit down into the armchair to read a book.
lukemassa – inessive (-massa/-mässä)
- Focus on the ongoing activity
- “to be in the process of reading”
→ Istun nojatuolissa lukemassa kirjaa.
I’m sitting in the armchair reading a book (I’m in the middle of it).

Your sentence uses lukemaan, so the image is: I go and sit down so that I can read.

Why is there no minä in the sentence? How do I know it means “I”?

In Finnish, the personal ending on the verb usually makes the subject pronoun unnecessary:

istu-n – I sit
istu-t – you (sg) sit
istu-u – he/she sits
istu-mme – we sit
etc.

Because istun already contains -n (1st person singular), minä is normally dropped:

Istun illalla… – I sit in the evening…

You would include minä when you want to emphasize the subject:

Minä istun illalla pehmeään nojatuoliin lukemaan kirjaa.
→ I (as opposed to someone else) sit in the evening…

But in neutral statements, omitting minä is standard and natural.

Can I change the word order? For example, is Illalla istun pehmeään nojatuoliin lukemaan kirjaa also correct?

Yes, that word order is correct and quite natural.

Istun illalla pehmeään nojatuoliin lukemaan kirjaa.
→ Neutral, starts from the verb.
Illalla istun pehmeään nojatuoliin lukemaan kirjaa.
→ Puts light emphasis on illalla (in the evening), as in:
“In the evening, I sit down in the soft armchair to read a book.”

Finnish word order is relatively flexible. The core pieces you shouldn’t break up are things like:

pehmeään nojatuoliin (adjective + noun together)
lukemaan kirjaa (verb + its object together is strongly preferred)

But overall you can move time expressions (illalla) and sometimes verb phrases around for emphasis.

Is there any important nuance in saying pehmeään nojatuoliin instead of just nojatuoliin?

Grammatically, pehmeään just agrees with nojatuoliin in case and number:

nojatuoliin – into the armchair
pehmeään nojatuoliin – into the soft armchair

Stylistically, Finnish often uses adjectives like pehmeä, mukava, oma, etc., to make the image richer or more personal:

omaan nojatuoliin – into my own armchair
mukavaan nojatuoliin – into a comfortable armchair

So pehmeään nojatuoliin creates that cozy, comfortable picture of the evening routine, but from a grammar perspective it’s just normal adjective–noun agreement in the illative case.