Learn Hardcore Finnish: Järvi on syvä.

järvi can mean a lake or the lake depending on context.
The same form is used: järvi on syvä can be understood as either The lake is deep or A lake is deep.

You determine definiteness from:

Previous context: if you have already mentioned a specific lake, it will be understood as the lake.
Situation: if both speakers see one particular lake, järvi will naturally mean the lake.

Why is the word order järvi on syvä and not something else? Is the word order fixed?

The neutral word order in Finnish is usually Subject – Verb – Complement, similar to English:

järvi (subject)
on (verb)
syvä (predicative adjective)

So järvi on syvä is the most natural, unmarked order.

You can change the order for emphasis or style, but it becomes less neutral:

Syvä on järvi. – more poetic or emphatic, like Deep is the lake.
On järvi syvä. – also stylistic/poetic, not normal everyday speech.

For basic statements, stick to järvi on syvä.

Why is syvä not changed in any way? Shouldn’t it agree with järvi somehow?

It is agreeing, but in this particular structure the agreement is simple.

In the sentence järvi on syvä:

järvi is in the nominative case (the basic dictionary form).
syvä is a predicative adjective describing the subject.
In basic copula sentences with olla (to be), the predicative adjective also appears in the nominative singular when it describes a singular subject in the nominative.

So:

järvi on syvä – The lake is deep.
talo on iso – The house is big.
mies on väsynyt – The man is tired.

They all use nominative singular for the adjective, matching a singular, nominative subject.

What is the infinitive of on, and how do other forms of this verb look?

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

The most common present tense forms are:

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

So järvi on syvä follows the same pattern as hän on väsynyt (he/she is tired): subject + on + description.

How do I make this sentence plural, like The lakes are deep?

To make it plural, you change both the noun and the adjective:

järvi on syvä – The lake is deep.
järvet ovat syviä – The lakes are deep.

Changes:

järvi → järvet (plural nominative)
on → ovat (3rd person plural of olla)
syvä → syviä (plural partitive, which is normally used here for a plural predicative adjective)

So in the plural, Finnish typically uses the partitive plural for adjectives in this type of sentence.

Why is it järvi on syvä and not something like järvi on syvää? I’ve seen adjectives ending with -ä in other forms.

Both forms can exist in Finnish, but they mean different things.

järvi on syvä – the normal description
- Means simply: The lake is deep.
- syvä is nominative, used for a basic, complete quality.
järvi on syvää – special, more restricted usage
- syvää is partitive singular.
- This can appear in contexts where you talk about the substance or nature of something, like:
  - Vesi on kylmää. – The water is cold (as a substance).
- With järvi on syvää, the sentence sounds unusual on its own; you’d more likely see partitive with a noun:
  - Järvessä on syvää vettä. – There is deep water in the lake.

For standard “X is adjective” statements about a countable thing, use nominative: järvi on syvä.

How would I say a deep lake instead of the lake is deep?

Now the adjective becomes attributive (directly modifying the noun), instead of being a predicative after on.

syvä järvi = a/the deep lake

Compare:

syvä järvi – a deep lake (adjective before noun)
järvi on syvä – the lake is deep (adjective after verb on)

Both use the nominative singular forms syvä and järvi in this simple case.

How do I say The lake is not deep in Finnish?

Finnish uses a special negative verb instead of just adding a separate word like not.

The pattern is:
subject + negative verb + main verb (in a special form) + adjective

For The lake is not deep:

järvi ei ole syvä

Breakdown:

järvi – the lake
ei – negative verb, 3rd person singular
ole – the connegative form of olla (used with ei)
syvä – deep

So:

järvi on syvä – The lake is deep.
järvi ei ole syvä – The lake is not deep.

How would I say The lake is very deep or quite deep?

You just add an adverb before the adjective:

järvi on erittäin syvä – The lake is very deep.
järvi on todella syvä – The lake is really deep.
järvi on tosi syvä – The lake is really/very deep (more colloquial).
järvi on melko syvä – The lake is quite/fairly deep.
järvi on aika syvä – The lake is pretty/quite deep (informal).

The structure stays the same: järvi + on + [degree word] + syvä.

How do I pronounce järvi and syvä, especially the letter ä and the j?

Pronunciation tips:

j in Finnish is like the y in English yes.
ä is a front vowel, similar to the a in English cat, but a bit purer and shorter.

So roughly:

järvi ≈ “yar-vi” (with ä like in cat, and rolled or tapped r; stress on the first syllable: JÄR-vi)
syvä ≈ “sy-va”
- y is like the French u in lune or the German ü in Tür (rounded version of i).
- Stress: SY-vä (always first syllable in Finnish).

All Finnish words are stressed on the first syllable, so JÄR-vi on SY-vä.

Why isn’t järvi capitalized, even though in English Lake can start with a capital letter sometimes?

Finnish capitalization rules are simpler than English:

Only the first word of a sentence and proper names are capitalized.
Common nouns like järvi (lake), talo (house), kissa (cat) are not capitalized in the middle of a sentence.

So:

Järvi on syvä. – correct
järvi on syvä. – wrong because the sentence must start with a capital letter, but järvi itself is not a proper noun.

If Järvi were the name of a place (a town or family name), it would then be a proper noun and capitalized, but that’s a different usage.

Is järvi always järvi, or does it change form in other sentences?

Finnish is highly inflected, so järvi changes form according to case and number.

Some common forms:

järvi – nominative singular (a/the lake)
järven – genitive singular (of the lake)
järveä – partitive singular (lake in partitive contexts)
järvessä – inessive (in the lake)
järveen – illative (into the lake)
järvestä – elative (out of/from the lake)
järvet – nominative plural (lakes)
järvissä – inessive plural (in the lakes), etc.

In järvi on syvä, we use the basic nominative form because it is the subject of the sentence.

Can I use järvi on syvä as a complete sentence by itself in real conversation, or would it sound strange?

Yes, järvi on syvä is a perfectly normal, complete sentence in everyday Finnish.

You might use it:

When describing a lake to someone:
- Älä ui kauas. Järvi on syvä. – Don’t swim far. The lake is deep.
When comparing lakes:
- Tuo järvi on matala, mutta tämä järvi on syvä. – That lake is shallow, but this lake is deep.

It’s short and simple, but fully natural.