Learn Hardcore Finnish: Tarkistan, että tämä dokumentti on oikea, ennen kuin lähetän sen sähköpostitse. - I check that this document is correct before I send it by email.

että is the standard conjunction meaning that when you introduce a subordinate clause that reports/checks a fact: Tarkistan, että… = I check that….
Finnish normally puts a comma before subordinate clauses introduced by conjunctions like että, koska, kun, jos, etc. So Tarkistan, että… is the usual punctuation.

Why is the verb tarkistan in the first person singular, and what tense is it?

tarkistan comes from tarkistaa (to check/verify). The ending -n marks 1st person singular (I).
It’s in the present tense, which in Finnish can mean either I check (habitually) or I’m checking (right now) depending on context.

Why is it tämä dokumentti and not tämän dokumentin?

Here tämä dokumentti is the subject of the clause tämä dokumentti on oikea (this document is correct), so it stays in the nominative (basic form).
You’d see tämän dokumentin (genitive) if it were an object or part of another structure, e.g. Tarkistan tämän dokumentin = I check this document (checking the document itself, as an object).

What does on oikea mean exactly, and why use oikea instead of something like oikein?

on oikea literally means is correct / is the right one. Here oikea is an adjective describing dokumentti.
oikein is an adverb meaning correctly (describing an action), e.g. Tein sen oikein = I did it correctly. You don’t use oikein after olla (to be) to describe a noun.

Why does Finnish repeat että + a whole clause instead of using something like an infinitive?

Finnish often prefers an että-clause when the content is a full statement: that the document is correct.
An infinitive alternative is possible in some contexts but changes the structure and sometimes the nuance. For example, with other verbs you might see something like varmistaa + clause as well. With tarkistaa, tarkistan, että… is very natural and clear.

What is the role of ennen kuin, and why is it two words?

ennen kuin is a fixed conjunction meaning before (literally before than). It introduces a subordinate clause describing what happens later:
… ennen kuin lähetän sen = … before I send it.
It’s commonly two words in Finnish and is treated as a conjunction phrase.

Why is there a comma before ennen kuin?

Because ennen kuin introduces a subordinate clause, Finnish punctuation normally uses a comma to separate the main clause from that subordinate clause:
Tarkistan … , ennen kuin lähetän …

Why does the verb lähetän also have the -n ending?

lähetän is from lähettää (to send). The -n again marks 1st person singular (I send).
Even though it’s inside a subordinate clause, Finnish still conjugates the verb normally for person and tense.

What does sen refer to, and why is it sen rather than se?

sen means it in the accusative/genitive form used for a total object (a complete, bounded action): send it (completely).
se is the subject form (it), while sen is used when it is the object of many verbs, including lähettää: lähetän sen.

Could this be lähetän sitä instead, and what would it change?

Yes, lähetän sitä is grammatically possible but it typically suggests an ongoing/partial action (the partitive object), like I’m in the process of sending it / sending some of it, or the action is unbounded.
For sending a document as a completed event, lähetän sen (total object) is the normal choice.

What does sähköpostitse mean, and how is it formed?

sähköpostitse means by email / via email.
It’s built from sähköposti (email) + the suffix -itse, which expresses by means of / via. It’s a common way to make “via X” expressions: puhelimitse (by phone), postitse (by mail).

Is sähköpostitse the same as sähköpostilla?

They’re very close in meaning.

sähköpostitse = via email (a set, adverb-like “method” expression)
sähköpostilla = literally with/by email (adessive case; also used for means)
Both can work, but sähköpostitse sounds especially “channel/method”-focused and is very common in formal Finnish.

Why does Finnish say dokumentti instead of asiakirja? Are they interchangeable?

Both can mean document, but:

dokumentti is a common, often slightly more general/modern loanword feel (and very common in everyday office/IT contexts).
asiakirja is often more official/legal-leaning: a formal document or record.
In many contexts they overlap, but the tone can shift.

Does the word order matter here? Could I move parts around?

Finnish word order is flexible, but changes emphasis. The neutral, natural order is close to what you have.
You could emphasize the timing or method, for example:

Ennen kuin lähetän sen sähköpostitse, tarkistan, että tämä dokumentti on oikea. (emphasizes before sending)
Tarkistan, että tämä dokumentti on oikea, ennen kuin lähetän sen. (less focus on email, more general)

Do I need to repeat tämä dokumentti? Could Finnish drop it like English sometimes does?

Finnish can drop pronouns/subjects in many contexts, but here tämä dokumentti is useful because it clearly states what is being checked for correctness.
If it’s already obvious from context, you might shorten:

Tarkistan, että se on oikea, ennen kuin lähetän sen sähköpostitse. (I check that it’s correct before I email it.)
But then you rely on context for what se/sen refer to.

Why is it että tämä dokumentti on oikea and not että tämä dokumentti olisi oikea?

on is straightforward is (a real check of correctness).
olisi is conditional (would be) and would suggest uncertainty/hypothetical framing, like checking whether it would be considered correct in some scenario. For a normal “verify it is correct” meaning, on is the expected choice.