Learn Hardcore Finnish: Kun skannaa dokumentin, kannattaa tarkistaa, että jokainen sivu on mukana. - When you scan a document, it’s worth checking that every page is included.

Finnish often uses an impersonal “zero person” construction to express a general “you/one/people” meaning without naming a subject.
Kun skannaa dokumentin literally is “When (one) scans the document,” i.e. “When you scan a document (in general).”
This is common in instructions, advice, and general statements.

What tense is skannaa and why is it used even though it refers to a general situation?

Skannaa is present tense (from skannata). In Finnish, the present tense is routinely used for:

general truths/habits (“when you do X…”)
instructions and advice contexts
So it doesn’t mean “right now”; it can mean “whenever/if you scan…”

Why is dokumentin in that form—what case is it?

Dokumentin is the genitive, but here it functions as the accusative (total object) form for many nouns (genitive-looking accusative).
With skannata (“to scan”), the object is typically “complete/whole” → “scan the whole document,” so Finnish uses the total object:

skannaan dokumentin = “I scan the (whole) document”
Compare:
skannaan dokumenttia (partitive) = “I scan some of the document / I’m scanning (process ongoing) / not necessarily all of it”

Could this be written with passive instead of the impersonal form?

Yes. You could say e.g. Kun dokumentti skannataan, kannattaa tarkistaa… = “When the document is scanned, it’s worth checking…”
Difference in feel:

Kun skannaa dokumentin… feels like direct general advice (“when you scan…”)
Kun dokumentti skannataan… sounds more detached/formal (“when the document is scanned…”)

What does kannattaa mean here, and why is it followed by another verb?

Kannattaa means “it’s worth / it’s advisable.” It commonly takes an infinitive:

kannattaa tehdä = “it’s worth doing / you should do”
So kannattaa tarkistaa = “it’s worth checking / you should check.”

Why is it kannattaa tarkistaa and not something like kannattaa tarkistat (“you check”)?

Because kannattaa doesn’t take a finite verb after it. It takes the A-infinitive (dictionary form) of the main action:

kannattaa + infinitive: kannattaa tarkistaa
You can add a person in other ways (e.g., sinun kannattaa tarkistaa = “you should check”), but the checked action still stays in the infinitive.

What is the role of että in että jokainen sivu on mukana?

Että introduces a subordinate clause meaning “that”:

tarkistaa, että… = “check that…”
So kannattaa tarkistaa, että jokainen sivu on mukana = “it’s worth checking that every page is included.”

What does mukana mean here? Is it a verb?

Mukana is not a verb; it’s an adverb/essive-like form meaning “along, included, with (it), present.”
on mukana is a common phrase meaning “is included / is there / is part of it.”
So jokainen sivu on mukana = “every page is included.”

Why is there a comma after dokumentin and another after tarkistaa?

Finnish punctuation uses commas to separate subordinate clauses:

Kun skannaa dokumentin, … → comma after the kun-clause
… kannattaa tarkistaa, että … → comma before the että-clause
This is very regular in Finnish and often more “mandatory” than in English.

Could jokainen sivu be replaced with kaikki sivut, and would it change the meaning?

Yes, both are possible:

että jokainen sivu on mukana = “that each/every page is included” (emphasizes checking page by page)
että kaikki sivut ovat mukana = “that all pages are included” (more collective)
Grammatically, note the agreement:
jokainen sivu on (singular + singular verb)
kaikki sivut ovat (plural + plural verb)