Learn Hardcore Norwegian: Han må nyse hele tiden når han støvsuger. - He has to sneeze all the time when he vacuums.

Inference: Han må være allergisk = He must be allergic (speaker’s conclusion).
Past: måtte (He had to) → Han måtte nyse.
“Doesn’t have to”: use trenger ikke å or behøver ikke å, not må ikke (which means “must not”).

Why is når used here instead of da?

når = when/whenever for general, present, future, or habitual time.
da = when for a single, specific event in the past. Here it’s a habitual situation, so når is correct.

Could I use mens instead of når?

Yes, with a slightly different emphasis:

når focuses on “whenever it happens.”
mens stresses simultaneity (“while”). Both are fine: Han må nyse hele tiden mens han støvsuger puts a bit more focus on the actions happening at the same time.

What’s the word order after når? Why han støvsuger and not støvsuger han?

In subordinate clauses (like after når), the finite verb does not move to second position. You keep Subject–Verb order:

når han støvsuger (subject first, then verb)

If you front the subordinate clause, the main clause still obeys V2:

Når han støvsuger, må han nyse hele tiden. (finite verb må is second in the main clause)

Where does ikke go? And does må ikke mean “doesn’t have to”?

With a modal, put ikke after the modal: Han må ikke nyse … = He must not sneeze (prohibition), not “doesn’t have to.”
To say he doesn’t sneeze: Han nyser ikke (hele tiden) når han støvsuger.
To say he doesn’t have to: Han trenger ikke å nyse (odd meaning here) or more naturally change the verb: Han trenger ikke å støvsuge.

Can I use alltid instead of hele tiden? Where does it go?

Yes. Use: Han må alltid nyse når han støvsuger.

alltid normally comes right after the finite verb; with a modal, after the modal: må alltid nyse. Avoid end placement: Han må nyse alltid … sounds odd.

What does støvsuger mean here — verb or noun?

Verb (present tense of å støvsuge, “to vacuum”): han støvsuger = “he vacuums.” As a noun, en støvsuger = “a vacuum cleaner,” støvsugeren = “the vacuum cleaner.” Context tells them apart.

How would I say this in the past?

Habitual past: Han måtte nyse hele tiden når han støvsugde.
One specific past occasion: Han måtte nyse hele tiden da han støvsugde. Forms:
måtte (past of må)
støvsugde (preterite), perfect participle usually har støvsugd (also accepted: har støvsuget)

What are the forms of nyse?

Infinitive: å nyse
Present: nyser
Past: nyste or irregular nøs (both accepted)
Perfect: har nyst

Is hele tiden a fixed expression? Do I need a preposition like i?

It’s a fixed adverbial meaning “all the time/constantly.” No preposition:

Correct: hele tiden
Not: i hele tiden (unless you literally mean “for the entire period,” which is rare and different).

What about hele tida vs hele tiden?

Both are standard Bokmål. Hele tiden is a bit more formal/neutral; hele tida is more colloquial. (In Nynorsk it’s heile tida.)

Could I say “every time” instead of når?

Yes: Han må nyse hver gang han støvsuger. That’s a bit more explicit than når about the repeated condition.

Any quick pronunciation tips for tricky sounds?

å in må is a long “aw” sound.
y in nyse is a front rounded vowel (like German ü): try saying “ee” while rounding your lips.
ø in støv is a mid rounded vowel (like French eu in “peu”).
g in suger is pronounced [g]. Stress: HAN må NY-se HE-le TI-den når han STØV-su-ger (main stress typically on the first content word in each chunk).