Learn Hardcore Norwegian: Jeg vet at hjernen arbeider saktere når kroppen er kald. - I know that the brain works more slowly when the body is cold.

at is a conjunction equivalent to English that. It introduces a subordinate (object) clause after a verb of thinking or knowing like vet. In Norwegian you normally keep at even if in speech people sometimes drop it.

Why is the word hjernen used instead of hjerne?

In Norwegian the definite article is attached as a suffix.

hjerne = “(a) brain” (indefinite)
hjernen = “the brain” (definite)
Here we talk about the brain in general, so we use the definite form hjernen.

Why does arbeider mean “works” or “is working”? Is there a progressive form?

arbeider is the present tense of å arbeide (“to work”). Norwegian does not have a separate progressive form, so the simple present covers both English works (habitual) and is working (ongoing). If you want a clearer progressive meaning you can say:

hjernen holder på å arbeide saktere (“the brain is in the process of working more slowly”).

Why is the adverb saktere used instead of sakte?

sakte = “slowly” (positive form)
saktere = “more slowly” (comparative)
Since the sentence says the brain works more slowly when the body is cold, you need the comparative saktere. To say just works slowly, you’d use arbeider sakte.

Could you say mer sakte instead of saktere? And what about langsommere?

mer sakte is understandable (“more slowly”) but saktere is the idiomatic comparative.
langsommere is the comparative of langsom (“slow”) used attributively, but you can also use it adverbially: arbeider langsommere.
Native speakers most often say saktere, though you will hear mer langsomt or langsommere in some contexts.

Why is når used here instead of da or hvis?

når = “when” for general or recurring situations (whenever).
da = “when” for a specific moment in the past.
hvis = “if” (conditional).
Since this is a general statement about whenever the body is cold, når is the correct choice.

What happens if you start the sentence with Når kroppen er kald?

You can front the subordinate clause, but you keep the normal subject–verb order inside it (no inversion), because når introduces a subordinate clause:
Når kroppen er kald, vet jeg at hjernen arbeider saktere.

Why is the adjective kald used instead of kaldt or kalda?

kroppen is a common-gender noun.
In predicative position (after er), adjectives use the indefinite strong form. For common gender that is uninflected: kald.
If the noun were neuter, you would say er kaldt.

Why don’t we need a comma before at in jeg vet at hjernen…?

In Norwegian commas before subordinate clauses introduced by at are optional and often omitted in informal or semi-formal writing. You may see a comma in very formal texts, but it’s not required.

What’s the difference between vet and kjenner in Norwegian?

vite (jeg vet) = to know a fact or piece of information.
kjenne (jeg kjenner) = to know/be familiar with a person, place or feel something (e.g. jeg kjenner deg = “I know you”).
Since the sentence states a factual truth, you need jeg vet.