Tarkistan, että sähköpostiosoite on oikein, ennen kuin lähetän viestin sähköpostitse.

Finnish typically separates a main clause from a subordinate clause with a comma. Here the subordinate clause starts with että (that), so you get:

• Tarkistan, että … = I check that …

The later comma before ennen kuin separates another subordinate clause:

• …, ennen kuin … = …, before …

että introduces a content clause (a “that-clause”):

• Tarkistan, että sähköpostiosoite on oikein = I check that the email address is correct

It’s usually not omitted in standard Finnish in this structure. In very casual speech you might sometimes hear it dropped in some contexts, but here että is the normal, clear choice.

In Finnish, oikein is an adverb meaning correctly / right, and it’s commonly used after olla in this kind of “is correct” statement:

• osoite on oikein = the address is correct

You can also say:

• osoite on oikea = the address is (the) correct one / is correct

Often:

• on oikein focuses on “it checks out / it’s correct as written”
• on oikea can feel a bit more like choosing the correct item among options

Both are possible; on oikein is very natural here.

Because it’s the subject of on:

• sähköpostiosoite (subject) + on (is) + oikein

Subjects with olla are normally nominative:

• X on Y = X is Y / X is …

Both tarkistan and lähetän are in the Finnish present tense, which covers several English options depending on context:

• tarkistan can mean I check / I’m checking / I will check
• lähetän can mean I send / I’m sending / I will send

Here it’s a general or immediate action sequence: I check … before I send …

ennen kuin literally works like before (the time when) and it introduces a subordinate clause with its own verb:

• ennen kuin lähetän viestin = before I send the message

Finnish typically uses a full clause after ennen kuin, rather than something like an -ing form.

viestin is the object form here (often analyzed as genitive/accusative singular depending on grammar tradition). With a total, complete action like “send a message,” Finnish commonly uses this “total object” form:

• lähetän viestin = I will send the (whole) message / a message

If you were emphasizing an incomplete/ongoing/partial action, you might see the partitive:

• lähetän viestiä = I’m sending a message (focus on the process / not presented as a completed whole)

In this sentence, the idea is a completed act after checking, so viestin fits well.

sähköpostitse means by email / via email (the method/channel). It’s a common adverb-like form for “by means of X.”

sähköpostilla (adessive) can also mean by email, but it can sometimes feel more concrete/instrument-like (“using email”), and in some contexts it can sound a bit less “set phrase” than -itse.

In practice:

• lähettää sähköpostitse = very standard “send via email”
• lähettää sähköpostilla = also possible, often similar meaning

Finnish usually doesn’t need subject pronouns because the verb ending shows the person:

• tarkistan = I check
• lähetän = I send

You can add minä for emphasis or contrast, but it’s not required:

• Minä tarkistan… = I (specifically) check…

The basic order here is very neutral and common, but Finnish word order is flexible for emphasis. For example:

• Ennen kuin lähetän viestin sähköpostitse, tarkistan, että sähköpostiosoite on oikein.
  = Before I send the message by email, I check that the email address is correct.

You can move the ennen kuin clause to the front to highlight the timing, but you still keep the commas separating the clauses.