Älä heitä lasia roskiin, vaan kierrätä se erikseen.

Älä is the negative imperative for 2nd person singular—in other words, don’t ...! when speaking to one person.

So:

• Heitä! = Throw it!
• Älä heitä! = Don’t throw it!

Finnish does not make negative commands by just adding a word like English not. Instead, it uses a special negative word:

• älä = don’t (singular)
• älkää = don’t (plural / formal)

So Älä heitä... literally works like Don’t throw...

Here heitä is the 2nd person singular imperative of heittää (to throw).

Basic verb:

• heittää = to throw

Imperative:

• heitä! = throw!

After älä, Finnish uses the same basic imperative-type form:

• Älä heitä = don’t throw

A useful extra note: heitä can also be a pronoun form meaning them in other contexts, but here it is clearly a verb, because it comes right after älä.

Because the sentence is negative, the object is in the partitive: lasia.

A very important Finnish rule is:

• In a negative sentence, the object is usually in the partitive.

So:

• Heitä lasi roskiin. = Throw the glass into the trash.
• Älä heitä lasia roskiin. = Don’t throw the glass / glass into the trash.

Here lasia is the partitive form of lasi.

There is also a second reason why lasia feels natural: glass is often treated as a material/substance in recycling contexts, and material words often appear in the partitive when talking about some amount or type of material.

In this kind of recycling sentence, it most naturally means glass as a material/category.

So the sentence sounds like the kind of instruction you might see on packaging or a sign:

• Don’t throw glass in the trash; recycle it separately.

However, grammatically, context matters. lasi / lasia can also refer to:

• a drinking glass
• glass material
• glass packaging/items made of glass

In this sentence, because of the recycling context, learners should usually understand lasia as glass in the general recycling sense.

Roskiin means into the trash / into the garbage.

It is the illative form, which often expresses movement into something.

Compare:

• roskat = trash, garbage
• roskiin = into the trash

This is a very common Finnish expression:

• heittää roskiin = throw away, throw into the trash

So even though English often says in the trash, Finnish uses a form that is closer to into the trash.

Because Finnish often treats trash/garbage as a plural-like collective idea: roskat.

So:

• roska = a piece of trash
• roskat = trash / garbage
• roskiin = into the trash

This is just how the idiom normally works. English speakers often want to analyze it as into the bin, but Finnish commonly says roskiin even when referring to throwing something away generally.

Because Finnish usually uses vaan after a negative first clause when the meaning is but rather / instead.

So:

• Älä heitä lasia roskiin, vaan kierrätä se erikseen. = Don’t throw glass in the trash, but rather recycle it separately.

This pattern is very common:

• ei X vaan Y
• älä X vaan Y

By contrast, mutta is the more general but. In this sentence, vaan is the natural choice because the second part replaces the first idea.

That is a very good question, because these words are related but not the same.

• kiertää = to go around, circulate, turn
• kierrättää = to make something circulate; in modern usage, to recycle

So the dictionary form here is:

• kierrättää = to recycle

Its 2nd person singular imperative is:

• kierrätä! = recycle!

That is why the sentence says:

• vaan kierrätä se = but recycle it

So you should learn kierrättää as the normal verb for to recycle.

Because in an imperative sentence, a total object often appears in the nominative form, and for this pronoun that form is se.

So:

• Kierrätä se. = Recycle it.

But in an ordinary indicative sentence, you would usually get:

• Minä kierrätän sen. = I recycle it.

This is one of those Finnish object-case patterns that learners meet again and again:

• in many normal statements: sen
• in imperatives: often se

So kierrätä se is exactly what you would expect in a command.

Erikseen means separately.

In this sentence it tells you that the glass should be:

• sorted separately
• collected separately
• not mixed with general trash

So kierrätä se erikseen means more than just recycle it. It suggests:

• put it in the proper separate recycling stream/container

This is very common in instructions about waste sorting.

Yes, absolutely.

• Älä heitä sitä roskiin = Don’t throw it in the trash.

That version sounds more like you are talking about one specific known thing.

The version with lasia is more category-based and instruction-like:

• don’t throw glass in the trash

So the difference is roughly:

• Älä heitä lasia roskiin = Don’t throw glass into the trash.
• Älä heitä sitä roskiin = Don’t throw it into the trash.

In real-life recycling language, the sentence with lasia sounds very natural because it names the material first, then refers back to it with se.

The word order here is very natural and standard, but Finnish is somewhat flexible.

This sentence is built as:

• Älä heitä
  • object + destination
• vaan kierrätä
  • object + adverb

So:

• Älä heitä lasia roskiin, vaan kierrätä se erikseen.

That is the most neutral order for an instruction.

You could change word order in Finnish for emphasis, but this version is the clearest and most typical for a notice or command.