Learn Hardcore Finnish: Kasvihuoneen ovi jätetään vähän auki, jotta ilma vaihtuu eikä kasveista tule liian kosteita. - The door of the greenhouse is left slightly open so that the air changes and the plants don’t become too damp.

Kasvihuoneen ovi jätetään vähän auki
≈ The greenhouse door is left a bit open
but more literally: People/one leave(s) the greenhouse door a bit open.

If you said:

Jätämme oven vähän auki = We leave the door a bit open.

That has an explicit subject me (we).
Jätetään is used when you want a general, impersonal instruction or description.

Why is there no separate word for is in ovi jätetään auki (like “is left open”)?

Finnish usually does not need an auxiliary verb like is in such constructions. The verb form jätetään already expresses:

the action (leave),
the passive/impersonal nature,
and the tense (present).

So:

ovi jätetään auki
literally combines door + is-left-open into one verb.

English separates this into is + left, but Finnish can do it with a single verb form. The state open is expressed by the adjective auki.

What is the role of vähän in vähän auki? Is it necessary?

Vähän means a little, a bit, slightly. It modifies auki (open):

auki = open
vähän auki = a bit open / slightly open

It is not grammatically necessary, but it adds meaning.

Ovi jätetään auki = The door is left open (no indication how much).
Ovi jätetään vähän auki = The door is left slightly ajar, not wide open.

You could also use hieman instead of vähän with very similar meaning.

Why is jotta used here instead of että or koska?

Jotta introduces a purpose clause: so that / in order that.

… jätetään vähän auki, jotta ilma vaihtuu …
= … is left a bit open so that the air will circulate …

Rough contrasts:

jotta = so that, expressing intended purpose or desired result.
että = that, more neutral, used for reported speech or factual results.
koska = because, used for reasons/causes, not intentions.

Here the focus is on why we do it on purpose (to ensure the air changes), so jotta is the natural choice.

Why is the verb after jotta in the normal present tense (ilma vaihtuu) and not in some special form like a conditional (vaihtuisi)?

Both are possible, but the nuance differs.

… jotta ilma vaihtuu
– neutral, often used for general truths or standard procedures.
– Here it means: this is what happens as a result of that practice.
… jotta ilma vaihtuisi
– more clearly a desired or hoped‑for result, slightly more hypothetical.

In everyday, generic instructions like this sentence, the plain present vaihtuu is very common and natural.

What does ilma vaihtuu literally mean, and why is ilma in the nominative?

Ilma vaihtuu literally means the air changes / gets exchanged / circulates.

ilma = air (subject, nominative)
vaihtua = to change, to be exchanged (intransitive verb, no object)

Because ilma is the thing that undergoes the change, it is the subject and appears in the nominative (basic) form.

If you wanted to say people change the air / ventilate the room, you could use a transitive verb:

vaihtaa ilmaa = to change air (ventilate)
ihmiset vaihtavat ilmaa = people are changing the air

Why is it eikä and not ja ei before kasveista tule liian kosteita?

Eikä is the fused form of ja (and) + ei (not), and it works like and not / nor.

… jotta ilma vaihtuu eikä kasveista tule liian kosteita.
= … so that the air circulates and the plants don’t become too damp.

In standard Finnish, when you connect a second clause that is also negative, you typically use eikä (or eikä myöskään):

Hän ei tullut, eikä soittanut.
= He did not come, and did not call.

Using ja ei is possible in some speaking styles, but eikä is the normal, smooth written form here.

Why is kasveista in the -sta / -stä form? What does it express here?

Kasveista is the elative case, usually meaning from / out of / from inside something.

With the verb tulla (to become), Finnish often uses the pattern:

jostakin tulee jokin/joksikin
= something becomes something / turns into something

So:

kasveista tulee liian kosteita
literally: from the plants comes too‑damp (ones)
idiomatically: the plants become too damp.

The idea is the plants (as they are now) → become → too damp.
The starting point (what is changing) goes into elative (kasveista).

Why is liian kosteita in the partitive plural and not something like liian kosteat?

Several things push it towards the partitive plural kosteita:

Verb of becoming
With verbs like tulla (to become), the new state is very often in the partitive, especially when it is about a degree or quality rather than a clear, fixed identity.
The adverb liian
Liian = too (much).
With qualities like temperature, wetness, etc., liian commonly takes a partitive predicative:
- Kahvi on liian kuumaa. = The coffee is too hot.
- Kasveista tulee liian kosteita. = The plants become too damp.
Plural subject
Kasvit are plural, so the adjective is plural too: kosteita (partitive plural).

You might sometimes see nominative forms with liian (liian kuuma, liian kostea), but in structures with tulla + degree of quality, the partitive is very typical and sounds natural here.

Could the sentence say Kasvihuoneen ovi pidetään vähän auki instead of jätetään vähän auki? What is the difference?

Yes, you could say:

Kasvihuoneen ovi pidetään vähän auki
= The greenhouse door is kept a bit open.

Nuance:

jätetään auki emphasises the action of leaving it open (you leave it that way when you go).
pidetään auki emphasises the state of being kept open (maintained open, not allowed to close).

In many practical contexts, both would be understood similarly, but jätetään suggests: you don’t close it fully; you leave it slightly open.

Why is there a comma before jotta?

In Finnish, independent clauses connected by jotta, koska, että, vaikka, etc., are usually separated by a comma.

Kasvihuoneen ovi jätetään vähän auki,
(first clause: what is done)
jotta ilma vaihtuu eikä kasveista tule liian kosteita.
(second clause: for what purpose / with what result)

So the comma marks the boundary between the main clause and the purpose clause introduced by jotta.

The verbs are in the present tense (jätetään, vaihtuu, tule); does this mean it is happening right now?

Not necessarily. Finnish uses the present tense for:

general truths
instructions and habitual actions
timeless descriptions

Here it describes a general practice:

Kasvihuoneen ovi jätetään vähän auki…
= As a rule / in this situation, the greenhouse door is left a bit open…

So it is more like a description of what one does (in general), not a single event happening right now.