Learn Hardcore Finnish: Tasainen katu ei ole vaarallinen.

Tasainen katu ei ole vaarallinen.
→ A flat street is not dangerous. (generic: any flat street)
In a specific context it could also mean
→ The flat street is not dangerous.

Whether you understand it as a or the (or as a general statement like “flat streets are not dangerous”) depends purely on context, not on any word in the Finnish sentence.

Why do we say ei ole and not something like ei on or just ei vaarallinen?

Finnish forms negation with a special negative verb ei, plus the main verb in a special “connegative” form.

The basic verb is olla (to be).
In the 3rd person singular:
- Positive: hän on / katu on (he is / the street is)
- Negative: hän ei ole / katu ei ole (he is not / the street is not)

So:

Katu on vaarallinen. – The street is dangerous.
Katu ei ole vaarallinen. – The street is not dangerous.

You normally cannot say *katu ei vaarallinen; you need ole there in a full sentence. (In very short answers you might hear Ei vaarallinen, but that’s an elliptical, “short” answer, not a full clause.)

What are the grammatical roles of tasainen, katu and vaarallinen here?

In Tasainen katu ei ole vaarallinen. we have:

tasainen – an adjective in nominative singular
katu – a noun in nominative singular (the subject)
ei ole – the negated verb “to be”
vaarallinen – an adjective in nominative singular, the predicative (it describes the subject)

Structure in simple terms:

[Tasainen katu] – “a flat street” (subject)
ei ole – “is not”
[vaarallinen] – “dangerous” (what the subject is / is not)

So the sentence literally matches English structure quite closely: A flat street is not dangerous.

Why are tasainen and vaarallinen in the basic (nominative) form and not something like tasaisella or vaarallista?

Two separate things:

Attributive adjective before a noun
- tasainen katu = a flat street
  Here tasainen is an attributive adjective. It must agree with katu in case and number.
- Nominative: tasainen katu
- If you changed the case, both would change:
  - tasaisella kadulla – on the flat street (adessive)
  - tasaisesta kadusta – from the flat street (elative)
In our sentence the subject is just plain nominative: tasainen katu.
Predicative after “to be”
- katu (ei ole) vaarallinen – the street (is not) dangerous
  The adjective vaarallinen is a predicative; in a simple sentence like this, it is normally in nominative as well.

The form vaarallista would be partitive, which you often see with objects in negative sentences, but here vaarallinen is not an object; it’s a description of the subject, so it stays nominative.

I learned that negation often causes the partitive case. Why isn’t vaarallinen in the partitive (like vaarallista) after ei ole?

That “negation → partitive” rule applies mainly to objects of negated verbs:

Näen koiran. – I see a dog.
En näe koiraa. – I don’t see (the/a) dog. (koira → koiraa, object becomes partitive)

In Tasainen katu ei ole vaarallinen, vaarallinen is not an object; it’s a predicative adjective describing the subject (tasainen katu).

For simple X is (not) Y sentences, the predicative usually stays in the nominative:

Katu on vaarallinen. – The street is dangerous.
Katu ei ole vaarallinen. – The street is not dangerous.

So negation does not force partitive here, because we are not dealing with an object but a subject complement.

Can the word order be different, like Katu ei ole tasainen vaarallinen or Katu ei ole vaarallinen tasainen?

Not freely. Normal, neutral word order here is:

Tasainen katu ei ole vaarallinen.
Subject (with adjective) – negated verb – predicative adjective.

Other natural orders (depending on context and emphasis) include:

Katu ei ole vaarallinen. – The street is not dangerous.
(Just dropping the adjective tasainen.)
Tämä katu ei ole vaarallinen. – This street is not dangerous.

But something like:

*Katu ei ole tasainen vaarallinen
*Katu ei ole vaarallinen tasainen

is ungrammatical or at least very wrong-sounding, because:

You normally place only one predicative adjective directly after ole.
Attributive adjectives (describing the noun directly) go before the noun: tasainen katu, not katu tasainen in this simple structure.

Word order in Finnish is flexible for information structure (topic, focus), but the roles must still be clear. In this sentence type, you keep:

[Subject (+ its adjectives)] – [verb / negated verb] – [predicative].

Does Tasainen katu ei ole vaarallinen talk about one specific street or streets in general?

By itself, it can be read either way:

Generic / general statement
- A flat street is not dangerous.
  This is the most natural reading if there is no context: a general rule about flat streets.
Specific street
In a context where both speaker and listener know which street they mean, it could be:
- The flat street is not dangerous (as opposed to the steep one).

Finnish often uses a singular, article-less noun for such general statements:

Koira on uskollinen eläin. – The dog / A dog is a faithful animal. (= dogs in general)
Tasainen katu ei ole vaarallinen. – A flat street is not dangerous. (= flat streets in general)

Context tells you whether it’s generic or about one particular street.

How would I say “Flat streets are not dangerous” in the plural?

You’d usually say:

Tasaiset kadut eivät ole vaarallisia.

Breakdown:

tasaiset kadut – flat streets
- tasaiset: nominative plural of tasainen
- kadut: nominative plural of katu
eivät ole – 3rd person plural negation of olla
vaarallisia – partitive plural of vaarallinen

Why vaarallisia and not vaaralliset?

Tasaiset kadut eivät ole vaarallisia. is the most natural way to express a general property (“they are (not) dangerous (as a type)”).
Tasaiset kadut eivät ole vaaralliset. is grammatically possible but tends to sound more contrastive or classificatory, like “those flat streets are not the dangerous ones.”

For everyday usage, vaarallisia in this kind of generic sentence is what you want.

What is the basic form of on / ole, and how do I say “I am not dangerous”, “You are not dangerous”, etc.?

The basic (dictionary) form of the verb on / ole is olla (to be).

Present tense affirmative:

minä olen – I am
sinä olet – you (sg) are
hän on – he/she is
me olemme – we are
te olette – you (pl) are
he ovat – they are

Present tense negative uses the negative verb ei + ole:

minä en ole – I am not
sinä et ole – you (sg) are not
hän ei ole – he/she is not
me emme ole – we are not
te ette ole – you (pl) are not
he eivät ole – they are not

With vaarallinen:

En ole vaarallinen. – I am not dangerous.
Et ole vaarallinen. – You are not dangerous.
Hän ei ole vaarallinen. – He/She is not dangerous.
etc.

How would I say this sentence in the past tense: “The flat street was not dangerous”?

Change on / ole (present) to oli / ollut (past):

Positive past: katu oli vaarallinen. – the street was dangerous.
Negative past: katu ei ollut vaarallinen. – the street was not dangerous.

So your full sentence:

Tasainen katu ei ollut vaarallinen.
→ The flat street was not dangerous.

Structure is the same; only the verb changes:

present: ei ole
past: ei ollut

How do you pronounce Tasainen katu ei ole vaarallinen?

Approximate pronunciation (capital letters show stressed syllables, long sounds marked with :):

TA-sai-nen KA-tu ei O-le VAA-ral-linen

Key points:

In Finnish, stress is always on the first syllable of each word: TAsainen, KAtu, EI, Ole, VAArallinen.
Double vowels are long:
- aa in vaarallinen is a long a (hold it: vaa-ral-linen).
Double consonants are also long:
- ll in vaarallinen is a long l (like holding the l a bit: vaaral-linen).
r is tapped or trilled.
All letters are pronounced; spelling is very close to the sound.

So said smoothly: TA-sai-nen KA-tu ei O-le VAA-ral-linen.