Learn Hardcore Finnish: Skanneri ei toimi toimistossa tänään, joten lähetän dokumentin sähköpostitse. - The scanner doesn’t work in the office today, so I will send the document by email.

Because skanneri is the subject of the clause (The scanner doesn’t work), and Finnish normally uses the nominative form for a basic subject.
You’d see skanneria (partitive) mainly in different meanings/structures, e.g. if you were talking about an indefinite amount of “scanner” (rare) or in some existential/quantity-type sentences, not here.

How does the negation ei toimi work? Why isn’t the main verb conjugated?

Finnish negation uses a special negative verb (ei) that conjugates for person/number, and the main verb appears in the connegative form (a form that doesn’t show person).
So:

(se) toimii = it works
(se) ei toimi = it doesn’t work

Here it’s 3rd person singular: ei + toimi.

What tense is toimi / lähetän? Does it mean “today” or “right now”?

Both are present tense forms:

ei toimi = doesn’t work / isn’t working
lähetän = I send / I’m sending

Finnish present tense often covers both “general present” and “right now,” and it can also refer to the near future when the context makes it clear (here, tänään + the situation implies you’ll do it now/today).

What case is toimistossa and why is it used?

Toimistossa is the inessive case (-ssa/-ssä), meaning in/inside a place:

toimisto = office
toimistossa = in the office

It answers “where?” in the sense of being located within that place.

Could I also say toimistolla instead of toimistossa?

Sometimes, yes, but it changes the nuance:

toimistossa (inessive) = in the office (inside the space)
toimistolla (adessive) = at the office (at the location; not necessarily inside)

In everyday Finnish, both can occur depending on what you want to emphasize.

Why is there a comma before joten?

joten means so / therefore, and it typically introduces a result clause. In Finnish, it’s normal to place a comma before joten when it links two independent clauses:

Skanneri ei toimi…, joten lähetän…

What’s the difference between joten and koska?

They point in opposite logical directions:

koska = because (gives the reason)
- Lähetän dokumentin sähköpostitse, koska skanneri ei toimi.
joten = so/therefore (gives the result)
- Skanneri ei toimi, joten lähetän…

Both can be correct; they just frame the relationship differently.

Why is it lähetän and not lähettää or something else?

lähetän is the 1st person singular present tense of lähettää (to send):

minä lähetän = I send / I’m sending

Finnish often drops the pronoun minä because the verb ending -n already shows it’s “I.”

Why is the object dokumentin (with -n) instead of dokumenttia?

dokumentin is a total object form (often called accusative/genitive-looking in singular), used when the action is seen as complete / bounded: you send the whole document.
dokumenttia (partitive) would suggest an unbounded/ongoing idea like sending “some of the document,” sending it “in general,” or emphasizing the process rather than completion.

What does sähköpostitse mean grammatically? Is it a case ending?

sähköpostitse means by email / via email. It’s an adverbial form built with -itse, which expresses means/method (roughly “by way of”).
It’s not one of the most common “core” location cases like -ssa, but it’s a very common fixed way to say “via X”:

postitse = by mail
puhelimitse = by phone
sähköpostitse = by email

A common alternative is sähköpostilla (“with/by email”), which is also widely used.

Finnish has no articles—how do I know if it’s “a scanner” or “the scanner”?

You infer it from context. Finnish doesn’t have a/the, so Skanneri can mean a scanner or the scanner depending on what’s already known in the conversation (e.g., the office scanner you both know about).

Is skanneri a loanword, and how is it pronounced?

Yes, skanneri is a loanword (from “scanner”). Pronunciation follows Finnish spelling rules:

skan-ne-ri (three syllables)
nn is a long consonant (held a bit longer than a single n)
stress is on the first syllable: SKANneri