Learn Hardcore Finnish: Yksi viiva on suora.

Finnish doesn’t have articles (a, an, the). Sometimes yksi can be used a bit like a/an, but it always keeps a feeling of “one specific” or “one single”.
If you just wanted to say The line is straight / A line is straight in a neutral way, you would more likely say:

Viiva on suora. – The line is straight. / A line is straight.

So:

With yksi → emphasis on there being one (single) line.
Without yksi → just “the line” in context, or generic “a line”.

Why is it yksi viiva, not yksi viivaa?

With the number 1 (yksi), the noun stays in the nominative singular, its basic dictionary form:

yksi viiva – one line
yksi koira – one dog
yksi talo – one house

With numbers 2 and above, the noun goes into partitive singular:

kaksi viivaa – two lines
kolme koiraa – three dogs
neljä taloa – four houses

So yksi viiva is exactly what Finnish grammar expects here.
Yksi viivaa would be grammatically wrong in this meaning.

Why are both viiva and suora in the basic form (no endings)?

Because of their roles in the sentence:

yksi viiva – the subject (nominative case)
on – the verb
suora – the predicative (a complement linked to the subject by olla, “to be”)

In Finnish, when the subject is in nominative, a predicative adjective (or noun) that describes it is also usually in nominative:

Viiva on suora. – The line is straight.
Tyttö on iloinen. – The girl is happy.
Auto on uusi. – The car is new.

So:

viiva – nominative singular (subject)
suora – nominative singular (predicative agreeing with the subject)

That’s why both appear in the same basic form.

Could I say Yksi viiva on suoraan instead of suora?

No. Here you need suora, not suoraan.

suora is an adjective: straight (describing a noun or a state)
suoraan is an adverb: straight / directly (describing how an action is done)

With the verb olla (to be) and a state, Finnish uses the adjective:

Viiva on suora. – The line is straight.
Tie on suora. – The road is straight.

You use suoraan with action verbs:

Mene suoraan eteenpäin. – Go straight ahead.
Katso suoraan minuun. – Look straight at me.

So in this sentence, suora is the correct form.

What exactly is on?

On is the 3rd person singular present form of the verb olla – to be.

Full present-tense paradigm of olla:

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

In Yksi viiva on suora, the subject is third person singular (yksi viiva), so the verb form is on.

The negative form is:

Yksi viiva ei ole suora. – One line is not straight.
(ei = negative verb, ole = negative form of olla)

Is the word order fixed? Could I say Suora on yksi viiva?

The neutral Finnish word order here is:

[Yksi viiva] [on] [suora].
Subject – Verb – Predicative

You can move elements around for emphasis, but the meaning often changes.

Suora on yksi viiva.
Literally: “The straight one is one line.”
This sounds unnatural here and suggests something like “The straight thing is a line,” which is not what you want.

For your meaning, the natural orders are:

Yksi viiva on suora. – One line is straight.
Or without yksi: Viiva on suora. – The line is straight. / A line is straight.

So for a learner: keep subject – on – complement unless you have a specific emphasis in mind.

How would I say “One of the lines is straight”?

For “one of the X”, Finnish usually uses yksi + elative plural:

Yksi viivoista on suora.
= One of the lines is straight.

Breakdown:

yksi – one
viivoista – “from (among) the lines” (viiva → viivat (plural) → viivoista (elative plural))
on suora – is straight

This makes it completely clear that you are talking about one member of a group of lines, not just “one line” in isolation.

How do I say “The line is straight” instead? Is there an article in Finnish?

Finnish has no articles (a, an, the). Context supplies that information.

Viiva on suora.
Depending on context, this can mean:
- The line is straight.
- A line is straight. (more general or context-free)

If you want to clearly signal “this particular line”, you often use a demonstrative:

Tämä viiva on suora. – This line is straight.
Se viiva on suora. – That line is straight.

So compared to Yksi viiva on suora:

Yksi viiva on suora. – One line (among others) is straight.
Viiva on suora. – The line is straight (or: a line is straight), with definiteness coming from context.

What’s the difference between suora viiva and viiva on suora?

They’re different structures:

suora viiva – adjective + noun (a noun phrase)
- Means a straight line.
- Used as subject or object:
  - Piirsin suoran viivan. – I drew a straight line.
viiva on suora – full sentence with a predicative
- Means The line is straight.
- Structure: subject (viiva) + verb (on) + predicative adjective (suora).

Your sentence Yksi viiva on suora uses the second structure: it’s making a statement about the state of the line, not just naming a type of line.

How do you pronounce Yksi viiva on suora?

Approximate pronunciation (using English-friendly hints):

Yksi – [ˈyksi]
- y like German ü or French u in tu (front rounded vowel)
- ks pronounced together, like in tacks
viiva – [ˈviːʋɑ]
- ii is a long i (hold it: vii-va)
- v is a soft v, almost like English v/w mix
- Stress on the first syllable: VII-va
on – [on]
- Short o, as in British English dog (but shorter)
suora – [ˈsuorɑ]
- uo is a diphthong, glide from u to o (suo-)
- Stress on the first syllable: SUO-ra

Overall rhythm: stress always on the first syllable of each word:

Ýksi Víiva on Súora