Lajittelen sekajätteen ja biojätteen aina erikseen, koska haluan, että taloyhtiön piha pysyy siistinä.

Lajittelen is the 1st person singular present tense of lajitella, which means to sort or to separate into categories.

So:

• lajittelen = I sort / I am sorting
• In this sentence, because of aina (always), it is understood as a habitual action: I always sort...

Finnish present tense often covers both simple present and present continuous, depending on context.

They are the objects of the verb lajittelen. In this sentence, they appear in the total object form, which in the singular often looks like the genitive ending -n.

So:

• sekajäte → sekajätteen
• biojäte → biojätteen

Very roughly, this gives the sense of sorting these waste categories properly/as complete categories. If you used the partitive forms sekajätettä and biojätettä, the sentence would feel more like focusing on some amount of waste or on the activity itself.

This is an area where Finnish object cases can be tricky, so it is very normal to ask about it.

These are common recycling terms in Finnish:

• sekajäte = mixed waste / general waste / non-recyclable household waste
• biojäte = biowaste / food and other compostable organic waste

Finnish often uses the singular for waste categories in a general sense, just as English can say biowaste or mixed waste.

Erikseen means separately.

It is an adverb, so it describes how the sorting is done:

• lajittelen ... erikseen = I sort ... separately

This is different from eri, which is usually an adjective meaning different or separate:

• eri astiat = different/separate bins
• erikseen = separately

So here Finnish needs the adverb erikseen, not the adjective eri.

Finnish often leaves out subject pronouns when they are clear from the verb ending.

Here, lajittelen already tells you the subject is I:

• lajittelen = I sort

You could say Minä lajittelen..., but minä would usually add emphasis or contrast:

• Minä lajittelen, mutta hän ei lajitele. = I sort it, but he/she doesn't.

So omitting minä is completely normal.

They introduce two different kinds of subordinate clauses:

• koska = because
• että = that

Structure:

• Lajittelen ... koska haluan, että taloyhtiön piha pysyy siistinä.
• I sort ... because I want the housing company’s yard to stay tidy.

So:

1. koska gives the reason: because I want...
2. että introduces what is wanted: that the yard stays tidy

This is very common in Finnish:

• Haluan, että... = I want (that)...

Because the subject of the second action is different.

In the main clause, the subject is I:

• haluan = I want

In the että clause, the subject is taloyhtiön piha:

• piha pysyy siistinä = the yard stays tidy

When the subject changes, Finnish often uses an että-clause:

• Haluan, että piha pysyy siistinä.
• I want the yard to stay tidy.

If the subject stayed the same, Finnish would often prefer an infinitive structure:

• Haluan lajittella jätteet oikein.
• I want to sort the waste correctly.

Taloyhtiö is a very common Finnish housing term. It refers to the housing company or housing association responsible for an apartment building or residential property.

So taloyhtiön piha means the shared yard/courtyard belonging to the building or housing association.

A very literal translation of taloyhtiö would not always sound natural in English, so depending on context it might be translated as:

• housing company
• housing association
• apartment building association
• sometimes just building or property management in freer translation

Taloyhtiön is the genitive singular of taloyhtiö.

The genitive here shows possession or close association:

• taloyhtiön piha = the housing company’s yard

Finnish usually shows this relationship by putting the possessor first and adding the genitive ending:

• talon ovi = the house’s door
• opettajan kirja = the teacher’s book
• taloyhtiön piha = the housing company’s yard

Unlike English, Finnish does not use an apostrophe.

Because with verbs like pysyä (to stay, remain), Finnish often uses the essive case to describe the state something remains in.

So:

• siisti = tidy
• siistinä = tidy / in a tidy state

Compare:

• Piha on siisti. = The yard is tidy.
• Piha pysyy siistinä. = The yard stays tidy.

This is a very important pattern in Finnish:

• pysyä hiljaisena = stay quiet
• säilyä ehjänä = remain intact
• pitää puhtaana = keep clean

They are close, but not the same.

• on siisti = is tidy
  This describes the current state.
• pysyy siistinä = stays/remains tidy
  This emphasizes that the tidy state continues.

So in this sentence, pysyy siistinä makes sense because the speaker is talking about sorting waste in a way that helps the yard continue to be tidy over time.

Aina means always, and erikseen means separately.

Here the order aina erikseen is natural and means something like always separately:

• Lajittelen sekajätteen ja biojätteen aina erikseen.

Finnish word order is fairly flexible, but word order can affect emphasis. This sentence uses a neutral, natural order:

• verb
• object
• frequency adverb
• manner adverb

You could also hear:

• Lajittelen aina sekajätteen ja biojätteen erikseen.

That is also grammatical, but the emphasis is slightly different.

Finnish normally uses a comma before a subordinate clause.

So here:

• comma before koska because it introduces a because-clause
• comma before että because it introduces a that-clause

That gives:

• ..., koska haluan, että taloyhtiön piha pysyy siistinä.

This is standard Finnish punctuation.