Learn Hardcore Norwegian: Appen er pålitelig, så jeg er fornøyd.

Because så here means “so/therefore” and links two independent clauses. In Norwegian, it’s standard to put a comma between two main clauses when they’re connected by så in this causal/resultative sense: Appen er pålitelig, så jeg er fornøyd. You could also write a period: Appen er pålitelig. Så jeg er fornøyd.

Should there be inversion after så? Why isn’t it …, så er jeg fornøyd?

When så works as a coordinating conjunction meaning “so/therefore,” it does not trigger inversion. The normal order is Subject–Verb: …, så jeg er fornøyd.
If you start a new sentence with Så as an adverb meaning “then/so,” you do get inversion: Appen er pålitelig. Så er jeg fornøyd.

Can I use derfor or fordi instead of så?

derfor = “therefore.” Prefer a new sentence (or a semicolon): Appen er pålitelig. Derfor er jeg fornøyd.
fordi = “because.” It flips the cause/effect: Jeg er fornøyd fordi appen er pålitelig. Writing Appen er pålitelig, fordi jeg er fornøyd changes the meaning and sounds odd.

Could I say …, og jeg er fornøyd instead?

Yes, it’s grammatical: Appen er pålitelig, og jeg er fornøyd. But og merely adds information, while så clearly marks a result. If you want a cause-effect feel, så (or a full stop + Derfor …) is better.

Do I need to say fornøyd med something? Should it be …, så jeg er fornøyd med den?

Jeg er fornøyd can stand alone (“I’m satisfied/pleased”) when the context is clear.
To be explicit, add med: Jeg er fornøyd med appen / med den.
With a clause, use med at: Jeg er fornøyd med at appen er pålitelig.

What’s the nuance of fornøyd compared with tilfreds or glad?

fornøyd is everyday, “pleased/satisfied.”
tilfreds is a bit more formal or neutral.
glad is “happy/glad,” expressing an emotional mood more than a sense of being satisfied with a result.
Plural/predicate example: Barna er fornøyde/tilfredse.

Is pålitelig the best word here? How does it differ from stabil or other options?

pålitelig = reliable/trustworthy (does what it should; you can rely on it).
stabil = stable (doesn’t crash or vary much).
til å stole på (phrase) = reliable: Appen er til å stole på.
Avoid tillitsfull here; it means “trusting,” not “trustworthy.”

Where does pålitelig come from, and why is it spelled that way (not pålitlig)?

It’s built on the verb å lite på (noe/noen) = “to rely on/trust,” plus the adjectival suffix -elig. Hence på-lit-e-lig → pålitelig. The form pålitlig is not standard.

What gender is app, and is Appen correct?

In Bokmål, app is commonly masculine: en app – appen. Some speakers use feminine (ei app – appa) in informal speech, but masculine is safest in writing. Neuter (et app) is wrong. A longer synonym is applikasjon (en).

How do adjective endings work here? Why er pålitelig and not something else?

Predicate adjectives (after er/blir) agree but often look like the base form: Appen er pålitelig. Huset er pålitelig (no extra -t for adjectives ending in -ig/-elig).
Attributive, indefinite: en pålitelig app, et pålitelig system (still no -t for -ig/-elig).
Attributive, definite/plural: den pålitelige appen, pålitelige apper.
With fornøyd, plural adds -e: Barna er fornøyde.

How is the sentence pronounced? Any tips for tricky sounds?

Approximate guide (Eastern Norwegian):

Appen ≈ “AHP-en” (short a; stress on first syllable).
er ≈ “air.”
pålitelig ≈ “paw-LEE-teh-lee” (the final g is weak/often not clearly pronounced; å like “aw” in “law”).
så ≈ “saw” with a long vowel.
jeg often sounds like “yai/yei.”
fornøyd ≈ “for-NOYD” (the øy is like “oy” in “boy” but with rounded lips; the final d is often very soft or silent).

Could I say Pålitelig er appen?

Normally no. Main clauses in Norwegian are verb-second, so you’d say Appen er pålitelig. Fronting Pålitelig is possible only for special emphasis or stylistic effect, and even then it’s uncommon.

Is it okay to drop the subject and say …, så er fornøyd?

No. Norwegian is not a pro-drop language; you need the subject pronoun: …, så jeg er fornøyd.

What’s the difference between så here and så … at or sånn at?

så (in your sentence) = “so/therefore,” a connector of result.
så … at / sånn at = “so … that,” expressing degree/result: Appen er så pålitelig at jeg er fornøyd. (sånn at is more colloquial; slik at is the formal equivalent.)

How would this look in Nynorsk?

A natural Nynorsk version is: Appen er påliteleg, så eg er nøgd.
Variants: nøgd/fornøgd both exist in Nynorsk.