Learn Hardcore Finnish: On järkevää verrata vain omaa suomen taitoa siihen, millainen se oli viime vuonna. - It is sensible to compare only your own Finnish skills to how they were last year.

On järkevää is an impersonal expression that literally means “(It) is sensible / reasonable”. Finnish often leaves out the dummy subject se (“it”) in general statements.

Se on järkevää verrata… = It is sensible to compare… (a bit more specific)
On järkevää verrata… = It is (generally) sensible to compare… (more general, like a rule or advice)

So the sentence is giving general advice: “It is sensible to compare only your own Finnish skills to what they were last year.”

Why is it järkevää and not järkevä?

Järkevä is the basic (nominative) form of the adjective: sensible / reasonable.

Here we get järkevää, which is the partitive singular. In Finnish, when you have a structure:

On + adjective (partitive) + infinitive verb

it usually expresses a general, abstract evaluation of an activity:

On hauskaa lukea. – It is fun to read.
On vaikeaa puhua suomea. – It is difficult to speak Finnish.
On järkevää verrata… – It is sensible to compare…

So järkevää is in the partitive because it functions as a predicate in this kind of impersonal “it is X to do Y” structure.

What form is verrata, and why is that form used here?

Verrata is the basic dictionary form of the verb, called the first infinitive in Finnish grammar.

The pattern is:

On järkevää + [1st infinitive]
On järkevää verrata… – It is sensible to compare…

Other examples:

On tärkeää oppia kielioppia. – It is important to learn grammar.
On mukavaa tavata uusia ihmisiä. – It is nice to meet new people.

So verrata is used because it’s the verb in an infinitive clause after on järkevää.

Why is it omaa suomen taitoa and not oma suomen taito?

Omaa suomen taitoa is in the partitive case, because it’s the object of the verb verrata (to compare).

The verb verrata typically takes:

partitive for the thing you are comparing (jotakin),
and illative for what you compare it to (johonkin).

Pattern: > verrata jotakin johonkin

In the sentence:

omaa suomen taitoa = “(your) own Finnish skill” (in partitive)
siihen = “to that” (illative form of se, “it”)

So:

verrata omaa suomen taitoa siihen
= to compare your Finnish skill to that

Therefore we need omaa suomen taitoa (partitive), not nominative oma suomen taito.

Why is omaa in this form, and why isn’t a possessive suffix like -si used (taitosi)?

Oma means “own” and behaves like an adjective. It has to agree in case and number with the noun it modifies.

Nominative: oma suomen taito
Partitive: omaa suomen taitoa

Since taitoa is partitive (as the object of verrata), oma also appears in the partitive: omaa.

About the possessive suffix:

suomen taitosi = your Finnish skill (with -si = “your”)
oma suomen taitosi = your own Finnish skill
omaa suomen taitoasi = the same but in partitive

In this sentence, the speaker is giving general advice (like to anyone), so they often just say omaa suomen taitoa without explicitly marking “your”. Context tells us it means “your own”. You could say:

On järkevää verrata vain omaa suomen taitoasi…

That would be grammatical and a bit more explicitly addressed to “you”.

What exactly does suomen taito mean? Why not suomen kieli?

Suomi = Finnish (the language / the country, depending on context)
suomen = genitive of suomi
taito = skill, ability
suomen taito = skill in Finnish / Finnish language skill

So omaa suomen taitoa means “one’s own skill in Finnish”.

If you said suomen kieli, that would just be “the Finnish language”. The sentence is about your ability, so taito is the right word.

What is vain doing here, and can it be placed somewhere else?

Vain means “only / just”. It’s limiting what is being compared:

verrata vain omaa suomen taitoa
= to compare only (your) own Finnish skills

Word order is fairly flexible, but the normal, neutral place for vain is directly before the word or phrase it limits.

verrata vain omaa suomen taitoa – only your own skills (not others’)
vain verrata omaa suomen taitoa – the only thing to do is compare your own skill (a bit different nuance)

In this sentence, the first meaning is intended, so vain is correctly before omaa suomen taitoa.

What does siihen refer to, and why is it in that form?

Siihen is the illative form of se (“it / that”):

se (nominative) – it
siinä (inessive) – in it
siihen (illative) – into it, to it

The verb verrata typically uses:

verrata jotakin johonkin
compare something to something

So siihen is “to that”, and it refers back to your Finnish skill as it was last year. The following clause explains what “that” is:

siihen, millainen se oli viime vuonna
to what it was like last year

So siihen is grammatically required by verrata and semantically refers to the earlier state of your own Finnish skill.

What is the function of millainen in siihen, millainen se oli viime vuonna?

Millainen means “what kind (of)” / “what sort (of)”.

In this sentence, it introduces a subordinate clause that describes the previous state of your Finnish skill:

millainen se oli viime vuonna
literally: what-kind (of) it was last year

Together:

siihen, millainen se oli viime vuonna
= “to what it was like last year”

So millainen works similarly to an English relative / interrogative word like “what kind (of)” in a clause.

Could you replace millainen se oli with something like millaista se oli? What’s the difference?

Both are possible in Finnish, but there is a nuance:

millainen se oli viime vuonna
Uses nominative millainen with se as a normal subject:
what kind (of thing) it was last year
millaista se oli viime vuonna
Uses millaista (partitive), describing the quality or state in a more “continuous/qualitative” way:
what it was like last year (more about the nature/quality)

In practice, in this context, both would be understood almost the same. The original millainen se oli is perfectly natural and maybe a bit more “categorical” (“what kind of skill it was”), but the meaning here is essentially “what it was like last year” either way.

Why does the clause say millainen se oli instead of repeating suomen taito?

Finnish often replaces a repeated noun with se (“it”) once the noun is clear from context.

First mention: omaa suomen taitoa – your Finnish skill.
Second mention: millainen se oli viime vuonna – what it was like last year.

So se = “that skill”. Repeating suomen taito would sound heavy and unnatural:

Very heavy: …siihen, millainen oma suomen taito oli viime vuonna.
Natural: …siihen, millainen se oli viime vuonna.

What case is vuonna in viime vuonna, and what does this phrase literally mean?

Vuonna is the essive singular of vuosi (year).

vuosi – year (nominative)
vuonna – “in (the) year”

Viime vuonna literally means “in the last year”, but idiomatically it’s just:

viime vuonna = last year

So:

millainen se oli viime vuonna
= what it was like last year

Is the overall word order fixed, or could you say this sentence in a different way?

The given order is very natural:

On järkevää verrata vain omaa suomen taitoa siihen, millainen se oli viime vuonna.

You could make small, still natural variations, e.g.:

On järkevää verrata omaa suomen taitoa vain siihen, millainen se oli viime vuonna.
(focuses “only” more on what you compare to)

But large changes would either sound unnatural or change the focus. The original order is a typical, idiomatic way to express this idea.