Maljakko on korkea.

• maljakko = a noun, the subject of the sentence
• on = the verb to be in the he/she/it is form
• korkea = an adjective, describing the subject

So the structure is:

subject + verb + adjective

This is a very common Finnish sentence pattern.

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

A few common forms are:

• olen = I am
• olet = you are
• on = he/she/it is
• olemme = we are
• olette = you all are
• ovat = they are

Here, the subject maljakko is singular, so Finnish uses on.

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

So maljakko can mean:

• a vase
• the vase

Which one is intended depends on context.

That is very normal in Finnish, and learners often need time to get used to it.

Here maljakko is the subject of the sentence, so it is in the nominative singular, which is the basic dictionary form.

In simple equational or descriptive sentences like this, the subject is very often in the nominative:

• Talo on suuri.
• Kirja on uusi.
• Maljakko on korkea.

Because it is a predicate adjective: it comes after on and describes the subject.

In this kind of sentence, the adjective is usually in the nominative singular when the subject is singular:

• Talo on suuri.
• Auto on nopea.
• Maljakko on korkea.

So korkea matches the singular subject maljakko.

Because this sentence needs the basic predicate-adjective form, korkea.

• korkean is a different case form, not the one used here.
• korkeaa can appear in other structures, but not as the normal adjective in a simple sentence like this.

For a beginner, the important rule is:

After on, when you are simply describing a singular subject, use the adjective in its basic form:

• maljakko on korkea
• kirja on mielenkiintoinen
• huone on pieni

Yes, but it means something slightly different in structure.

• korkea maljakko = a tall vase / the tall vase
  
  • here korkea comes before the noun and directly modifies it
• Maljakko on korkea = The vase is tall
  
  • here korkea comes after on and works as a predicate adjective

So both are correct, but they are different kinds of phrases/sentences.

The most neutral word order is:

Maljakko on korkea.

Finnish word order is more flexible than English, but changing it usually changes the emphasis.

For example:

• Maljakko on korkea. = neutral statement
• Korkea on maljakko. = unusual, marked emphasis; not the normal beginner version

So yes, other orders are possible in some contexts, but the standard form to learn first is subject + on + adjective.

A simple learner-friendly guide:

mal-jak-ko

A few important points:

• Stress is on the first syllable: MALjakko
• The kk is a long consonant in Finnish, so it is held a little longer than a single k
• Finnish pronunciation is usually very regular compared with English spelling

So try something like: MAL-yak-ko, with a clearly longer kk

A simple guide is:

KOR-ke-a

Important points:

• Stress is on the first syllable: KORkea
• The ending -ea is pronounced as two vowel sounds, not one merged English-style sound

So it is roughly: KOR-ke-ah

Try to keep the vowels clear and separate.

In normal standard Finnish, yes.

If you want to say that something is something or is some quality, you normally use olla:

• Maljakko on korkea.
• Talo on vanha.
• Kirja on hyvä.

So unlike some languages where to be can be omitted, standard Finnish usually keeps it in sentences like this.

Yes. In a sentence like this, the predicate adjective normally agrees in number with the subject.

So:

• Maljakko on korkea. = singular
• Maljakot ovat korkeita. = plural

Notice that both the verb and the adjective change:

• on → ovat
• korkea → korkeita

That is a very useful pattern to learn early.

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

It is short, but Finnish often uses short simple sentences very naturally. This makes it a good beginner example because it shows a very common pattern clearly:

noun + on + adjective