Learn Hardcore Finnish: Minä juon vettä siksi että se on terveellistä.

Minä juon veden.
you would mean I drink the water (all of it) – a specific, delimited amount, like one glass/bottle that you finish completely.

Plain vesi (nominative) as an object (Minä juon vesi) would be ungrammatical.

So vettä is used because you are talking about water in general / some water, not “the whole water of some specific container”.

Why does terveellistä end in -stä? Why not terveellinen?

The base form is the adjective terveellinen (healthy, healthful).
In the sentence, we have:

se on terveellistä

Here terveellistä is in the partitive singular. There are two main ideas to know:

After olla (to be), the “description word” (predicative) can be nominative or partitive in Finnish.
When you talk about a substance or a general property, Finnish often uses the partitive predicative.

Compare:

Vesi on terveellistä.
Water is (something) healthy / healthful (as a substance in general).
Tämä vesi on terveellinen.
This water is healthy (e.g. not dirty, safe to drink).

So in se on terveellistä, we are talking about water as a substance having the quality of being healthy, so terveellistä (partitive) is very natural.

What exactly does siksi että mean, and why do we need both words?

Siksi että is a two-part conjunction that means roughly for the reason that.

Breakdown:

siksi = for that reason, therefore
että = that (conjunction introducing a clause)

Together:

Minä juon vettä siksi että se on terveellistä.
≈ I drink water for the reason that it is healthy.

If you used only siksi:

Minä juon vettä siksi.
= I drink water for that reason (but you don’t say what the reason is).

If you used only että here, it would be ungrammatical, because että needs something in front that explains the point of the että-clause (siksi, niin, etc., or a verb like tiedän että…).

What is the difference between siksi että and koska?

Both can express a reason, but they feel a bit different:

koska ≈ because
Minä juon vettä, koska se on terveellistä.
= I drink water because it is healthy.
siksi että ≈ for the reason that, for the purpose that
Minä juon vettä siksi että se on terveellistä.
Literally: I drink water for that reason, that it is healthy.

In practice:

koska is the most common everyday choice for “because”.
siksi että can sound a bit more formal or emphatic, highlighting the reason itself: that is the reason why I do it.

For most beginner situations, you can safely use koska when you mean because.

Does juon mean “I drink” or “I am drinking”? How do I know?

Juon is the present tense, 1st person singular of juoda (to drink).

Finnish does not have a separate progressive form like English I am drinking. The same form juon covers both:

I drink (habit, general)
I am drinking (right now)

So Minä juon vettä can mean either, depending on context:

Talking about habits:
Minä juon vettä siksi että se on terveellistä.
→ I drink water (as a habit) because it’s healthy.
Talking about right now (e.g. with a time word):
Nyt minä juon vettä.
→ Now I am drinking water.

Context and extra words (like aina, nyt, usein) tell you which reading is intended.

Can I drop minä and just say Juon vettä siksi että se on terveellistä?

Yes.

In Finnish, the personal ending on the verb already shows the subject:

juo-n = I drink
juo-t = you drink
hän juo = he/she drinks

So:

Minä juon vettä siksi että se on terveellistä.
Juon vettä siksi että se on terveellistä.

Both are grammatically correct and mean the same. In neutral everyday speech and writing, Finns often omit the pronoun unless they want to emphasize it (for contrast, clarity, or style).

Using minä is completely fine for learners, but it will sound a bit more “spelled-out” / emphatic.

What does se refer to, and why is it se and not some other pronoun?

In this sentence:

se refers back to vesi / vettä (water).

Finnish has:

hän = he / she (for people, in standard language)
se = it (for things, and also for people in informal spoken language)

Here, water is a thing, so se is used:

Minä juon vettä siksi että se on terveellistä.
= I drink water because it is healthy.

So se = it (the water).

Why is there no word for “a” or “the” in this sentence? How do I know if it’s “water”, “some water” or “the water”?

Finnish has no articles (a/an, the). The ideas that English expresses with articles are spread across:

case endings (like vettä vs veden),
context,
optional words like eräs, tämä, se, etc.

In this sentence:

Minä juon vettä…
→ usually understood as I drink water or I drink some water, because vettä is partitive (indefinite amount, not specific).

If you wanted “the water (a specific amount, all of it)”, you would probably say:

Minä juon sen veden.
= I drink that water. / I drink the water (that we both know about).

So by default, Juon vettä is interpreted as I drink (some) water, with a general, non-specific meaning.

Could the word order be different, like Siksi juon vettä että se on terveellistä?

Finnish word order is fairly flexible, but not all theoretically possible orders sound natural.

The most natural versions here are:

(Minä) juon vettä siksi että se on terveellistä.
Siksi (minä) juon vettä, että se on terveellistä. (with a comma, and slightly different emphasis)

General tendencies:

Putting siksi at the beginning (Siksi juon vettä…) puts strong emphasis on the reason: For that reason I drink water…
The version Juon vettä siksi että se on terveellistä is a very neutral, everyday word order.

Beginners are usually safest with:

(Minä) juon vettä, koska se on terveellistä.
(Minä) juon vettä siksi että se on terveellistä.

Should there be a comma before että in this sentence?

In standard written Finnish, että usually starts a subordinate clause, and you normally put a comma before it:

Minä juon vettä siksi, että se on terveellistä.

Many style guides recommend the comma here. In real texts, you may see both with and without:

siksi, että (with comma)
siksi että (without comma)

For a learner, the important point is the grammar and meaning; you can safely write:

Minä juon vettä siksi, että se on terveellistä.

and it will be considered correct in standard Finnish.