Learn Hardcore Finnish: Roskis on tyhjennettävä ja roskapussi vietävä ulos ennen yötä. - The trash can must be emptied and the trash bag taken outside before night.

Finnish doesn’t use a separate word like must or have to here; the combination olla + -tava/-ttava participle itself carries that meaning of necessity.

What grammatical form is tyhjennettävä, and how is it formed from tyhjentää?

Tyhjennettävä is the passive present participle of the verb tyhjentää.

Base verb: tyhjentää (to empty)
Passive present (finite verb): tyhjennetään (is/are emptied)
Passive present participle: tyhjennettävä

Formation pattern:

Take the passive stem: tyhjennete- / tyhjennettä-
Add the participle ending -vä
→ tyhjennettävä

So structurally:

tyhjentää → tyhjennetään → tyhjennettävä

This -tava/-ttava participle has several uses in Finnish, but in this sentence it’s part of the necessive construction, especially with olla:

roskis on tyhjennettävä = the bin must be emptied
kirja on luettava = the book must be read
raportti on kirjoitettava = the report must be written

Why is there no separate word like täytyy or pitää (must, have to) in this sentence?

Because the construction olla + -tava/-ttava already expresses the idea of must / has to.

So:

Roskis on tyhjennettävä
≈ Roskis täytyy tyhjentää
≈ Roskis pitää tyhjentää

All three mean roughly “The bin must be emptied.” The differences are mostly in style:

on tyhjennettävä
- More neutral/objective, slightly formal or instructional.
- Common in written instructions, rules, notices.
täytyy / pitää tyhjentää
- Very common in spoken Finnish.
- Sounds like a more personal or conversational obligation.

So the sentence doesn’t “need” a word like täytyy, because on tyhjennettävä already fills that role.

Who is supposed to empty the bin and take the bag out? Where is the subject?

There is no explicit subject in this sentence; that’s the point of the construction.

Roskis on tyhjennettävä is an impersonal / passive-like necessive:

It means: “The bin must be emptied (by someone).”
The doer is left unspecified.
It can mean “you” (the reader), “we”, “they”, or just “people in general”, depending on context.

Similarly:

roskapussi vietävä ulos
≈ “the trash bag must be taken out (by someone)”

Finnish often avoids naming a subject when:

the doer is obvious from context (e.g. a note on the fridge),
or it’s a general rule.

Compare:

Roskis tyhjennetään joka ilta.
= The bin is emptied every evening. (passive, doer not stated)
Roskis on tyhjennettävä joka ilta.
= The bin must be emptied every evening. (necessity + no stated doer)

In both, the actor is implicitly “someone” or “whoever is responsible”.

Why is roskis in the basic form (nominative) and not in some other case like roskista?

In this construction, roskis behaves grammatically like a subject, so it appears in the nominative (basic form):

Roskis on tyhjennettävä.
The bin must be emptied.

Structurally, it’s like:

Substantive in nominative
- olla
  - -tava/-ttava participle

Examples:

Ovi on suljettava. = The door must be closed.
Ikkunat ovat pestävät. = The windows must be washed.

Even though in English “the bin” is the object of “empty”, in Finnish this pattern treats it more like the thing that has the property of “being something that must be emptied” – so it takes the subject-like nominative.

Partitive (roskista) would not be used here, because:

the idea is to empty the whole bin (a complete action), not “some of the bin”;
and this specific construction normally uses nominative for this role.

What is the difference between roskis and roskapussi in this sentence?

They refer to two different things:

roskis
= the trash can / bin itself (the container).
roskapussi
= the trash bag / garbage bag that is inside the bin.

So the sentence is saying:

The container must be emptied.
The bag must be taken outside.

In practice, emptying the roskis usually means removing the roskapussi from it and taking that roskapussi ulos.

Why isn’t there another on before roskapussi vietävä ulos?

The second on is simply omitted because it would be repetition of the same auxiliary verb.

The full, more explicit version would be:

Roskis on tyhjennettävä ja roskapussi on vietävä ulos ennen yötä.

In Finnish, when two clauses share the same verb (here, on) and are connected with ja (and), it’s common and natural to drop the repeated verb in the second part:

Roskis on tyhjennettävä ja roskapussi vietävä ulos.
(the second on is understood)

This is especially common in written Finnish and in slightly formal or instructional language.

What form is vietävä, and why isn’t it viedä here?

Vietävä is, like tyhjennettävä, a passive present participle in -tava/-ttava form.

Base verb: viedä (to take, to carry somewhere)
Passive present: viedään (is/are taken)
Passive present participle: vietävä

Formation (simplified):

Stem vie-
Add -tävä (with normal vowel changes)
→ vietävä

Why not viedä?
Because in the olla + -tava/-ttava construction, you must use this participle form, not the infinitive:

roskapussi on vietävä (correct)
the trash bag must be taken

Not:

roskapussi on viedä (incorrect in standard Finnish)

So vietävä matches tyhjennettävä structurally:

roskis on tyhjennettävä
roskapussi (on) vietävä ulos

Why do we use ulos instead of something like ulkona or ulko?

Finnish distinguishes clearly between movement and location with different forms:

ulos = out(wards), movement to outside
- viedä ulos = to take out
- mennä ulos = to go out
ulkona = outside, location
- roskapussi on ulkona = the trash bag is outside
ulkoa = from outside (movement from)
- tuoda jotain ulkoa = bring something from outside

In this sentence, viedä (to take, carry [to somewhere]) expresses movement, so the correct form is:

roskapussi vietävä ulos
≈ the trash bag must be taken out

Why is it ennen yötä and not ennen yö or ennen yön?

Because the preposition ennen (before) almost always takes the partitive case.

ennen yötä
- yötä = partitive singular of yö (night)

Examples of the same pattern:

ennen iltaa (before the evening) – ilta → iltaa
ennen joulua (before Christmas) – joulu → joulua
ennen lomaa (before the holiday) – loma → lomaa

So:

ennen yötä = before night / before nighttime

Forms like ennen yö or ennen yön are not standard in this meaning; ennen simply wants the partitive after it in normal modern Finnish.

Can the word order be changed, for example Roskis on ennen yötä tyhjennettävä?

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

These are natural:

Roskis on tyhjennettävä ja roskapussi vietävä ulos ennen yötä.
Roskis on tyhjennettävä ennen yötä ja roskapussi vietävä ulos.
Ennen yötä roskis on tyhjennettävä ja roskapussi vietävä ulos.

Your variant:

Roskis on ennen yötä tyhjennettävä

is grammatically possible, but sounds a bit unusual or heavy in ordinary language. Typically, time expressions like ennen yötä are placed:

at the beginning for emphasis: Ennen yötä roskis on tyhjennettävä…, or
after the verb phrase: …on tyhjennettävä ennen yötä.

So yes, movement is possible, but the original or these alternatives sound more natural.

How could I say the same idea in a more casual, everyday way?

Some more casual alternatives:

Using täytyy:
- Roskis täytyy tyhjentää ja roskapussi täytyy viedä ulos ennen yötä.
Using pitää:
- Roskis pitää tyhjentää ja roskapussi pitää viedä ulos ennen yötä.
Direct imperative (very common in speech and notes at home):
- Tyhjennä roskis ja vie roskapussi ulos ennen yötä.
  = Empty the bin and take the trash bag out before night.

The original sentence with on tyhjennettävä / vietävä sounds a bit more formal or instructional, like something you might see in written rules or official instructions.