Forelesningen er i den største forelesningssalen på universitetet.

The ending -en marks the definite singular form of a masculine (or sometimes common gender) noun in Norwegian.

• en forelesning = a lecture (indefinite singular)
• forelesningen = the lecture (definite singular)

In this sentence, we are talking about a specific lecture that speaker and listener both know about, so the definite form forelesningen (the lecture) is used.

No, that would be ungrammatical in standard Norwegian.

When you have:

• a definite noun (forelesningssalen – the lecture hall)
• modified by a superlative adjective (største – biggest / largest)

you must use double definiteness:

den største forelesningssalen
literally: the biggest the-lecture-hall

Structure:

• den – definite article
• største – superlative adjective
• forelesningssal-en – noun with definite ending

So you need both den and the -en ending: den største forelesningssalen.

Two things are going on here:

1. Superlative form of the adjective

2. Definite noun phrase structure

3. For the adjective stor (big):

   • Comparative: større (bigger)

   • Superlative: størst / største (biggest)

   • størst is the predicative form (used after to be):
     
     • Hun er størst. – She is the biggest.
   • største is the attributive form (used before a noun):
     
     • den største forelesningssalen – the biggest lecture hall

   So before a noun, you use største, not størst.

4. The noun itself must also be in the definite form:

   • en forelesningssal – a lecture hall
   • forelesningssalen – the lecture hall
   • den største forelesningssalen – the biggest lecture hall

So den største forelesningssalen is the correct full form.

Forelesningssalen is a compound noun:

• forelesning – lecture
• sal – hall
• forelesningssal – lecture hall
• forelesningssal-en – the lecture hall

Norwegian (like German and other Scandinavian languages) often joins nouns together into one long word, instead of using something like “lecture hall” with two separate words. The linking -s- in forelesningssal is very common in compounds.

In Norwegian, the preposition often used for being inside a room or enclosed space is i (in).

• i forelesningssalen – in the lecture hall (inside the room)
• i klasserommet – in the classroom
• i huset – in the house

på is more like on / at (and is used for many locations and institutions), but for an actual room you are physically in, i is the normal choice.

So:

• Forelesningen er i den største forelesningssalen. – correct
  The lecture is in the largest lecture hall.

When talking about institutions in Norwegian (schools, universities, hospitals, workplaces), på is often used to mean at:

• på universitetet – at the university
• på skolen – at school
• på jobben – at work
• på sykehuset – at the hospital

So here på universitetet means at the university (as an institution/place), not literally “inside the building”.

If you really meant inside the physical building as an object, you could use i, but in everyday language på universitetet is the natural phrase.

Universitet is a neuter noun in Norwegian.

Neuter nouns typically form the definite singular with -et, not -en:

• et universitet – a university (indefinite singular)
• universitetet – the university (definite singular)

Compare:

• Masculine: en forelesningssal → forelesningssalen
• Neuter: et universitet → universitetet

So universitetet is simply “the university” in its correct definite neuter form.

Yes, that is also grammatical, and it’s something you might hear.

Both are fine:

• Forelesningen er i den største forelesningssalen på universitetet.
• Forelesningen er på universitetet i den største forelesningssalen.

The difference is just a slight emphasis:

• First version focuses first on the room, then adds where that room is (at the university).
• Second version focuses first on the university, then specifies which room there.

Both are natural; context and preference decide which sounds better.

You’d make forelesningen plural and adjust the verb:

• Forelesningene er i den største forelesningssalen på universitetet.

Breakdown:

• forelesningene – the lectures (definite plural)
• er – are
• The rest stays the same, because we still have one largest lecture hall.

You’d make forelesningssal plural definite:

• i de største forelesningssalene

Structure:

• de – the (plural definite article)
• største – superlative adjective (same form in plural)
• forelesningssal-ene – the lecture halls (definite plural)

So:

• i de største forelesningssalene på universitetet
  = in the largest lecture halls at the university

Norwegian often uses a simple present form of “to be” (er) and lets context do the rest.

• Forelesningen er i den største forelesningssalen …
  literally: The lecture is in the largest lecture hall …

This is fully natural and usually understood as “takes place in / is held in”.
You can explicitly say:

• Forelesningen holdes i den største forelesningssalen …
  = The lecture is held in the largest lecture hall …

But in everyday speech and writing, “Forelesningen er i …” is perfectly normal and often preferred for its simplicity.