Kansallispuisto on suuri.

Kansallispuisto is a compound noun:

• kansallinen = national
• puisto = park

In the compound, the first part appears as kansallis-, so the full word literally looks like national-park.

Finnish uses compound words very often, much more than English does, so one long word is very normal.

Finnish does not have articles like English a/an and the.

So Kansallispuisto on suuri can mean:

• A national park is big
• The national park is big

Which one is intended depends on context. This is very normal in Finnish.

On is the third person singular form of the verb olla, which means to be.

So:

• minä olen = I am
• sinä olet = you are
• hän on = he/she is
• se on = it is

In this sentence, on simply means is.

Here, suuri is a predicative adjective: it comes after the verb on and describes the subject.

In this basic sentence, both words are in the nominative:

• kansallispuisto = nominative singular
• suuri = nominative singular

So suuri stays in its dictionary form.

Compare:

• Kansallispuisto on suuri. = The national park is big.
• suuri kansallispuisto = a big national park

In both of these, the adjective is singular nominative.

Yes, adjectives usually agree with the noun in number and often also in case.

For example:

• suuri kansallispuisto = a big national park
• suuret kansallispuistot = big national parks

And in other cases:

• suuressa kansallispuistossa = in a/the big national park

So agreement is an important feature of Finnish grammar.

This is the most neutral Finnish word order for a simple statement:

• subject + verb + complement

So:

• Kansallispuisto = subject
• on = verb
• suuri = complement/adjective

Finnish word order is more flexible than English word order, but different orders usually change the emphasis.

For example:

• Kansallispuisto on suuri. = neutral statement
• Suuri on kansallispuisto. = unusual, poetic, or strongly marked
• Onko kansallispuisto suuri? = Is the national park big?

So the sentence you have is the normal basic order.

A helpful rough guide is:

kan-sal-lis-pui-sto

A few pronunciation points:

• k is always like English k in skin, never like s
• a is a clear ah
• ll is a longer l
• ui is a diphthong, said together
• stress is on the first syllable: KANsallispuisto

Finnish stress is very regular: it normally falls on the first syllable.

A rough guide is:

suu-ri

Important points:

• uu is a long vowel, so hold it slightly longer
• r is typically rolled or tapped
• stress is on the first syllable: SUUri

In Finnish, vowel length matters. A short vowel and a long vowel can change the word.

It is singular.

The sentence is talking about one national park in a general or context-based way.

The plural would be:

• kansallispuistot = national parks

So:

• Kansallispuisto on suuri. = A/The national park is big.
• Kansallispuistot ovat suuria. = National parks / The national parks are big.

Finnish adjectives change form depending on grammar.

Here, the sentence is a basic X is Y sentence, so the adjective appears in the basic nominative form:

• suuri = big

Other forms appear in other structures. For example:

• suuren kansallispuiston = of a/the big national park
• suuressa kansallispuistossa = in a/the big national park
• isoa kansallispuistoa can appear in some specific grammatical contexts, but not in this simple sentence

So suuri is the correct basic form here.

Yes. Kansallispuisto is just a noun meaning national park, and if you refer back to it, you would normally use se in standard Finnish for it:

• Kansallispuisto on suuri. Se on kaunis myös.
• The national park is big. It is beautiful too.

It is true that in everyday spoken Finnish, se is also often used for people, but here that is not especially relevant. For things like parks, se is the normal pronoun.

Yes. On is the present tense form of olla here.

So the sentence means that the national park is big now, or is big as a general fact.

If you wanted the past tense, you would say:

• Kansallispuisto oli suuri. = The national park was big.

Yes. It is a completely natural, grammatical Finnish sentence.

It sounds like a simple textbook-style example because it is short and clear, but real Finnish absolutely allows this kind of sentence. In actual conversation, it would usually appear in a context, for example when describing a place:

• Kävimme siellä eilen. Kansallispuisto on suuri.
• We went there yesterday. The national park is big.

So it is both correct Finnish and a useful beginner pattern.