Learn Hardcore Finnish: Poistan pudonneet lehdet kukkapenkistä ja vien ne kompostiin kasvihuoneen taakse. - I remove the fallen leaves from the flower bed and take them to the compost behind the greenhouse.

In this sentence, the speaker is removing all the relevant leaves from the flowerbed. Finnish uses the nominative plural for a total, complete object in this kind of context:

Poistan pudonneet lehdet.
= I remove the fallen leaves (all of them, a definite set).

If the action was partial, ongoing, or indefinite, you’d use lehtiä (partitive):

Poistan pudonneita lehtiä.
= I remove some fallen leaves / I’m removing (an unspecified number of) fallen leaves.

So lehdet here suggests a bounded, whole group of leaves that are being completely removed.

Could I say pudonneita lehtiä instead of pudonneet lehdet? What would change?

Yes, you can, but the meaning changes:

pudonneet lehdet
- grammatically: adjective (NOM pl) + noun (NOM pl)
- meaning: the fallen leaves (seen as a whole group; total object).
pudonneita lehtiä
- adjective in partitive plural
  - noun in partitive plural
- meaning: some fallen leaves / fallen leaves in general (partial or indefinite).

So:

Poistan pudonneet lehdet kukkapenkistä.
= I remove the (all of the) fallen leaves from the flowerbed.
Poistan pudonneita lehtiä kukkapenkistä.
= I (am) remove(ing) some fallen leaves from the flowerbed (not necessarily all).

The original sentence emphasizes a complete clean-up.

What case is kukkapenkistä, and what does the ending -stä mean?

kukkapenkistä is in the elative case (ending -sta / -stä), which usually means “out of / from inside / from”.

Breakdown:

kukka = flower
penkki = bench, bed
kukkapenkki = flowerbed (compound noun)
kukkapenkistä = from the flowerbed

So kukkapenkistä expresses the source of the removal:

Poistan lehdet kukkapenkistä.
= I remove the leaves from (out of) the flowerbed.

Compare:

kukkapenkissä (inessive, -ssa/-ssä) = in the flowerbed
kukkapenkistä (elative, -sta/-stä) = from the flowerbed

Why is kukkapenkki one word? Could I say kukka penkki?

kukkapenkki is a compound noun:

kukka (flower) + penkki (bench/bed) → kukkapenkki (flowerbed).

In Finnish, these kinds of combinations are normally written as one word, and only the last part takes the case ending:

basic form: kukkapenkki
from the flowerbed: kukkapenkistä
in the flowerbed: kukkapenkissä

Writing kukka penkki as two words would be read as “a flower and a bench” or something similarly odd, not as “flowerbed”.

What tense are Poistan and vien, and can this sentence also refer to a regular habit?

Both poistan and vien are in the present tense. Finnish present tense covers:

actions happening right now
regular/habitual actions
near-future actions (in context)

So this sentence can mean:

“I am removing the fallen leaves … and taking them to the compost …” (right now)
“I (always/usually) remove the fallen leaves … and take them …” (habitual, if context suggests routine)

The context decides whether we understand it as “right now” or “usually”.

What is the difference between vien and tuon?

Both are about moving something, but from different viewpoints:

viedä → vien = to take (away from the speaker’s current location)
tuoda → tuon = to bring (towards the speaker’s location or some reference point)

In the sentence:

vien ne kompostiin
= I take them to the compost (I carry them away from where I am now).

You would use tuon in a context like:

Tuon lehdet kompostiin, jos haluat.
= I’ll bring the leaves to the compost (towards the listener’s or agreed location).

Why is it ne and not niitä after vien?

ne and niitä are both forms of the 3rd person plural pronoun (“they / them”), but in different cases:

ne = nominative plural, also used for total objects in many contexts
niitä = partitive plural, used for partial/indefinite objects

Here, ne refers back to pudonneet lehdet as a whole, definite group:

vien ne kompostiin
= I take them (the leaves, all of them as a set) to the compost.

If you used niitä, it would imply some of them or an unbounded quantity, which would clash with pudonneet lehdet in the first part that already presents them as a specific, total group.

What case is kompostiin, and how is that form built?

kompostiin is in the illative case, which typically means “into / to (inside)”.

komposti = compost (the compost heap / bin / area)
kompostiin = into the compost

The illative often shows movement into something. The common pattern is:

vowel-ending word: talo → taloon (into the house)
i-stem word like komposti: komposti → kompostiin (into the compost)

So vien ne kompostiin = “I take them to/into the compost”.

What does kasvihuoneen taakse literally mean, and why is kasvihuoneen in the genitive?

kasvihuoneen taakse breaks down as:

kasvihuone = greenhouse
kasvihuoneen = genitive form (“of the greenhouse”)
taakse = “to behind” (illative form of takana/taakse set)

Together: kasvihuoneen taakse = (to) behind the greenhouse.

In Finnish, many location words work as postpositions (they come after the noun), and the noun goes in the genitive:

kasvihuoneen takana = behind the greenhouse (static location)
kasvihuoneen taakse = (to) behind the greenhouse (movement to that place)

So kasvihuoneen is in the genitive because taakse behaves like a postposition that requires the noun in genitive.

What is the difference between takana, taakse, and takaa?

These are related location forms of taka- (“back, behind”):

takana = behind, static location
- Auto on talon takana. = The car is behind the house.
taakse = (to) behind, movement towards a position behind
- Menen talon taakse. = I go behind the house.
takaa = from behind, movement away from a position behind
- Tulen talon takaa. = I come from behind the house.

In your sentence:

kasvihuoneen taakse = (to) behind the greenhouse (movement to that location).

Why is the phrase kompostiin kasvihuoneen taakse in that order? Could I say kasvihuoneen taakse kompostiin instead?

Both orders are grammatically possible, but there is a nuance:

vien ne kompostiin kasvihuoneen taakse
- more natural: “I take them to the compost behind the greenhouse.”
- It sounds like there is a compost located behind the greenhouse (one place).
vien ne kasvihuoneen taakse kompostiin
- can be understood, but the order is a bit odd and less natural.
- It might sound more like two separate steps: first “behind the greenhouse”, then “to the compost”, or just stylistically clumsy.

Finnish tends to place more specific, defining locations later, so kompostiin kasvihuoneen taakse nicely reads as “to the compost (which is) behind the greenhouse.”