Learn Hardcore Norwegian: Studentene liker forelesningssalen fordi stolene er myke. - The students like the lecture hall because the chairs are soft.

studenter = students (indefinite plural, some students)
studentene = the students (definite plural, a specific group already known in the context)

In this sentence, we are talking about a particular group (for example, the students in this class), so Norwegian uses the definite plural studentene, just like English uses the students.

Why is the verb liker and not something like liker vs liker (different forms)? Does it change with the subject?

Norwegian verbs do not change according to person or number in the present tense.

jeg liker – I like
du liker – you like
han / hun liker – he / she likes
vi liker – we like
dere liker – you (plural) like
de liker – they like

So with studentene (they), the present tense is still simply liker. There is no -s form like English likes.

Why is forelesningssalen written as one long word?

Norwegian often creates compound nouns by joining words together without spaces.

Here:

forelesning = lecture
sal = hall

First you make a compound: forelesningssal = lecture hall
Then you make it definite singular (the lecture hall):

masculine noun sal → definite singular salen
compound: forelesningssal → forelesningssalen = the lecture hall

So forelesningssalen literally means the lecture-hall. Writing it as three separate words would be incorrect in Norwegian.

Why is it forelesningssalen and not en forelesningssal?

Again, this is definiteness.

en forelesningssal = a lecture hall (indefinite)
forelesningssalen = the lecture hall (definite)

The English translation also uses the: The students like *the lecture hall...
Norwegian expresses that *the-meaning by attaching -en to the noun (for masculine and some feminine nouns): salen.

What is the gender of sal, and how does that affect the forms?

sal is a masculine noun in Norwegian Bokmål. The basic forms are:

indefinite singular: en sal – a hall
definite singular: salen – the hall
indefinite plural: saler – halls
definite plural: salene – the halls

When it is part of a compound:

forelesningssal – lecture hall
forelesningssalen – the lecture hall

The masculine gender is what gives you -en in salen / forelesningssalen.

Why is it stolene and not just stoler?

Same definiteness rule as with studentene and forelesningssalen.

For stol (chair), which is masculine:

en stol – a chair
stolen – the chair
stoler – chairs
stolene – the chairs

In the sentence, we are talking about the chairs in that particular lecture hall, not just chairs in general, so Norwegian uses the definite plural stolene.

Why is the adjective myke and not myk?

Norwegian adjectives agree with the noun in number (singular/plural) and sometimes gender.

Basic forms of myk (soft):

masculine/feminine singular: myk – en myk stol (a soft chair)
neuter singular: mykt – et mykt sete (a soft seat)
plural (all genders): myke – myke stoler (soft chairs)

Since stolene is plural, you must use the plural form of the adjective: myke.
So: stolene er myke = the chairs are soft.

Why is the word order stolene er myke and not er stolene myke after fordi?

In Norwegian main clauses, the verb usually comes in the second position (V2 word order):

Stolene er myke. – The chairs are soft.

But after a subordinating conjunction like fordi (because), you are in a subordinate clause, where the normal order is Subject – Verb – (Object / Adjective), not V2:

fordi stolene er myke – because the chairs are soft

If you said fordi er stolene myke, it would sound like a question and would be grammatically wrong in this context.

Do I need a comma before fordi in Norwegian?

The full, more standard written form would usually be:

Studentene liker forelesningssalen, fordi stolene er myke.

In modern everyday writing, some people omit the comma in short sentences, but traditional and formal rules say:

Put a comma between the main clause and the subordinate clause that starts with fordi when the subordinate clause comes after the main clause.

So it is good practice to include the comma when you write.

Can I change the order and start with the fordi-clause?

Yes. You can say:

Fordi stolene er myke, liker studentene forelesningssalen.
(Because the chairs are soft, the students like the lecture hall.)

Notice two things:

The order inside the fordi-clause stays stolene er myke (Subject–Verb).
In the main clause that follows a fronted subordinate clause, Norwegian keeps the verb in second position:
liker studentene forelesningssalen, not studentene liker forelesningssalen.

What is the difference between fordi and på grunn av in a sentence like this?

Both can express reason, but they work differently grammatically:

fordi introduces a clause (has subject + verb):
Studentene liker forelesningssalen fordi stolene er myke.
på grunn av is a preposition that takes a noun or noun phrase:
Studentene liker forelesningssalen på grunn av de myke stolene.
(The students like the lecture hall because of the soft chairs.)

You cannot say på grunn av stolene er myke; after på grunn av you need a noun phrase, not a full clause.

Could I just say ... fordi de er myke instead of ... fordi stolene er myke?

Grammatically, yes:

Studentene liker forelesningssalen fordi de er myke.

Here de would normally refer back to the nearest plural noun, stolene, so most readers would understand it as because they (the chairs) are soft.

However, using stolene again is very clear and not at all unnatural in Norwegian, especially in an example sentence meant for learners. In normal conversation, a native speaker might very well use de to avoid repetition.

Could I use klasserommet instead of forelesningssalen?

You could, but the meaning changes slightly:

klasserommet = the classroom
forelesningssalen = the lecture hall (usually a bigger room, e.g., at a university)

So:

Studentene liker klasserommet fordi stolene er myke.
(The students like the classroom because the chairs are soft.)

Both are grammatically fine; they just describe different kinds of rooms.