Modal particles: jo, vel, nok, da …

Small words like jo, vel and nok don't change what you say, but how it comes across. They make spoken Norwegian sound natural.

We teach Bokmål, the written standard most people in Norway use.

In short: Jo, vel, nok and da don't change the content, only the tone. They show what you think the other person knows or believes.

Rules

Jo: Something you both know or should know: Det sa jeg jo! (I told you so!) Du har jo bil – you've got a car, after all.
Vel (assumption): «I suppose»: Du er vel sliten nå? (You must be tired?) Han kommer vel snart.
Vel (objection): Mild doubt or pushback: Det er vel ikke så farlig? (Surely it's not that bad?)
Nok: Probability, «probably»: Det ordner seg nok. Also a concession: Det er nok riktig, men vi har ikke råd.
Da: Softens or strengthens, mostly in speech: Kom igjen, da! (Come on!) Det er da ikke så vanskelig. Ha det bra, da!
Nå: Impatience or a reservation: Nå må du gi deg! (Give it a rest!) Det var nå ikke så ille.
Visst: «Apparently», «from what I hear»: Han har visst flyttet til Bergen.
Altså: Summing up or emphasis: Møtet er altså avlyst (So the meeting is cancelled). Det var altså helt utrolig!
Liksom: Spoken filler, «like», or irony: Han skulle liksom være ekspert. Avoid it in formal writing.
Egentlig: Softens questions or shows contrast: Hva mener du egentlig? Jeg skulle egentlig trene i dag (I was actually supposed to work out).
Position: Particles usually come right after the verb, before ikke: Han er jo ikke her.
Style: Use them freely in conversation and in emails to colleagues, but sparingly in formal texts.

Typical mistake: Reading visst as «certainly». Han kommer visst = apparently he's coming, not «he's definitely coming».