Learn Hardcore Norwegian: Nettverket på skolen er tregt i dag.

The base noun is nettverk (network), which is neuter in Norwegian.
Indefinite singular: et nettverk (a network).
Definite singular: nettverket (the network). The ending -et marks the definite form of neuter nouns.

Why is it på skolen and not i skolen or ved skolen?

på skolen = at the school (as an institution/place you attend or work at). This is the natural choice here.
i skolen can mean inside the school building physically, but more often refers to the school system as a domain (e.g., problems “in education”). It’s not used for “at school” in everyday talk.
ved skolen = by/near the school (next to the building), not what you want here.

Why is it tregt and not treg?

Adjectives agree with the grammatical gender/number of the noun when used after a linking verb like er:

Masculine/feminine singular: treg (En PC er treg.)
Neuter singular: tregt (Et nettverk er tregt. → Nettverket er tregt.)
Plural: trege (PC-ene er trege.)

Because nettverk is neuter, the predicate adjective is tregt.

Can I move i dag to the front? What happens to word order?

Yes: I dag er nettverket på skolen tregt. Norwegian main clauses follow the V2 rule: the finite verb (er) must be in second position. If you front i dag, the verb comes next, then the subject.

Is i dag written as one word or two?

Two words: i dag. The same goes for i går (yesterday) and i morgen (tomorrow). Writing idag/imorgen/igår is nonstandard in Bokmål.

What’s the difference between treg, sakte, and langsom here?

treg/tregt/trege describes something as sluggish/slow in performance. Most natural for computers, networks, apps.
sakte is an adverb (“slowly”): use it with action verbs. Example: Nettverket går sakte.
langsom/langsomt/langsomme is also “slow” but is more general/literary; you can say forbindelsen er langsom, but treg is what people usually say about tech.

Could I say Nettet på skolen er tregt i dag or use Internett instead?

Nettet på skolen er tregt i dag is very common and natural; nettet colloquially means “the internet/connection.”
Nettverket stresses the local network/Wi‑Fi infrastructure.
Internett is often used without an article. Internett er tregt i dag would sound like “the Internet (in general) is slow today,” which is a bigger claim than you intend. Internettet (with article) is old-fashioned.

Why are both nettverket and skolen in the definite form?

Each noun is a specific, known entity in context: “the network” and “the school.” Norwegian freely uses definite forms on both nouns in a X på Y (“X at Y”) structure: Nettverket på skolen.

Is Skolens nettverk er tregt i dag also correct? What about a compound like skolenettverket?

Skolens nettverk er tregt i dag is correct and a bit more formal/written.
A compound like skolenettverket is possible in Norwegian, but it sounds heavier and is less common unless it’s a set term/name. Everyday speech prefers nettverket på skolen.

How do you pronounce the tricky parts, especially tregt?

tregt: the cluster tends to sound like “trekt” (long e, and the g is effectively devoiced before t).
skolen: roughly “SKOO-len” (with a long oo).
nettverket: stress on the first syllable “NETT-…”.
i dag: “ee DAHG” (long a in dag). Don’t overthink it—clear vowels and stress carry you far.

Can I say Nettverket på skolen går tregt i dag?

Yes. gå + tregt/sakte is idiomatic for performance: Det går tregt/sakte = It’s running slowly. Here it means the same as er tregt.

Why er and not blir?

er states a current condition: it is slow (now/today).
blir means “becomes/gets”: Nettverket blir tregt om ettermiddagen (The network gets slow in the afternoons).

How do I negate it or ask a question?

Negation: Nettverket på skolen er ikke tregt i dag.
Yes/no question (verb first): Er nettverket på skolen tregt i dag?
Wh-question: Hvorfor er nettverket på skolen tregt i dag?

How can I intensify or tone it down?

Intensify: veldig, skikkelig (colloquial), kjempe- (as a prefix), altfor (too/overly).
- Nettverket er veldig/skikkelig tregt i dag.
- Nettverket er altfor tregt i dag.
Soften: litt, ganske.
- Nettverket er litt/ganske tregt i dag.

What are the comparative and superlative forms of treg?

Comparative: tregere (The network is slower = Nettverket er tregere)
Superlative (predicative): tregest (This one is slowest = Denne er tregest)
Attributive superlative takes an extra -e: det tregeste nettverket (the slowest network).