Stóllinn í bílnum er mjúkur en stóllinn í stofunni er mýkri.

Because Icelandic usually puts the definite article on the end of the noun instead of using a separate word.

• stóll = chair / seat
• stóllinn = the chair / the seat

The same thing is happening in bílnum and stofunni too, although there the article is combined with case endings.

The dictionary forms are bíll and stofa. In this sentence they appear as bílnum and stofunni because they come after í meaning in, and here í is expressing location, which normally takes the dative case.

So:

• í bílnum = in the car
• í stofunni = in the living room

They are also definite, so the forms include both case marking and the definite article.

With í, Icelandic often makes a location vs. movement distinction through case:

• í bílnum = in the car
• í bílinn = into the car

And similarly:

• í stofunni = in the living room
• í stofuna = into the living room

A good rule of thumb is:

• dative for location
• accusative for motion into / toward

Because mjúkur is a predicate adjective after er. In Icelandic, predicate adjectives normally use the strong form, even when the noun is definite.

Compare:

• mjúki stóllinn = the soft chair
• stóllinn er mjúkur = the chair is soft

So the sentence uses the normal predicative form: er mjúkur.

Because the adjective agrees with stóllinn, which is masculine singular nominative.

That gives:

• masculine: mjúkur
• feminine: mjúk
• neuter: mjúkt

Since stóll is masculine, mjúkur is the correct form here.

Mýkri is the comparative form of mjúkur, so it means softer.

This is not just a simple ending change. The stem changes too:

• mjúkur = soft
• mýkri = softer

So Icelandic comparatives are sometimes a bit irregular-looking, and this is a good example of that.

Yes. It is still describing stóllinn. But comparative adjectives often look less obviously gender-marked than the basic adjective form.

So:

• mjúkur clearly shows a masculine singular ending
• mýkri is the comparative form used here as the predicate adjective

The important point for a learner is that mýkri means softer and fits naturally with stóllinn er ...

Because Icelandic often repeats the noun where English might use the one.

English:

• the chair in the car is soft, but the one in the living room is softer

Icelandic often prefers:

• stóllinn ... en stóllinn ...

You could also say ... en sá í stofunni er mýkri, meaning ... but the one in the living room is softer, but repeating the noun is perfectly normal and very clear.

Literally, stóll means chair. But depending on context, it can be understood as seat.

In a car context, many speakers might more naturally say:

• sæti = seat
• bílsæti = car seat

Still, stóllinn í bílnum is understandable, especially in a teaching sentence.

Stofa usually means living room, sitting room, or lounge. It does not normally mean just any room.

So í stofunni specifically means in the living room.

En means but. It connects two contrasting main clauses.

The word order is normal:

• Stóllinn í bílnum = the subject phrase
• er = the verb
• mjúkur = predicate adjective

Then again:

• stóllinn í stofunni = subject phrase
• er = verb
• mýkri = predicate adjective

So the sentence has a very standard Icelandic structure. The phrase í bílnum or í stofunni is just part of the noun phrase telling you which chair is being talked about.