Omavastuu on sata euroa.

Omavastuu is a common Finnish word in insurance and payment contexts. It means the part you pay yourself before an insurer or some other payer covers the rest. In British English, excess is often the closest term; in American English, deductible is often the closest term.

It is a compound word:

• oma = own
• vastuu = responsibility, liability

So the idea is roughly your own share of the responsibility/cost.

Because Finnish very often forms compound nouns as a single word.

So:

• oma
  • vastuu → omavastuu

This is very normal in Finnish. English often writes similar ideas as two words, but Finnish usually joins them:

• sairausvakuutus = health insurance
• luottokortti = credit card
• matkalippu = travel ticket

If you are an English speaker, one of the most useful habits to build in Finnish is: when two nouns or noun-like elements belong tightly together, Finnish often makes them into one word.

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

So Finnish simply says:

• Omavastuu on sata euroa.

English needs something like:

• The deductible is one hundred euros. or
• A deductible of one hundred euros applies.

Finnish leaves article-type meanings to context. Whether something is definite or indefinite is usually understood from the situation, not from a separate word.

Because after numbers greater than one, Finnish usually puts the noun in the partitive singular.

So:

• yksi euro = one euro
• kaksi euroa = two euros
• viisi euroa = five euros
• sata euroa = one hundred euros

That is why the sentence has:

• sata euroa

and not:

• sata euro

This is one of the most important number patterns in Finnish.

Because sata euroa and sadan euron are used in different structures.

In your sentence:

• Omavastuu on sata euroa.

Here sata euroa is the amount stated after on. This is the normal counting form:

• numeral in its basic form
• noun in partitive singular

But sadan euron is a genitive phrase used to modify another noun:

• sadan euron omavastuu = a 100-euro deductible
• sadan euron maksu = a 100-euro payment

So compare:

• Omavastuu on sata euroa. = The deductible is 100 euros.
• Tässä vakuutuksessa on sadan euron omavastuu. = This insurance has a 100-euro deductible.

It is in the nominative, the basic dictionary form.

That is because omavastuu is the subject of the sentence:

• Omavastuu = subject
• on = verb
• sata euroa = predicate expressing the amount

So the structure is basically:

• X on Y = X is Y

Since omavastuu is the subject, its basic nominative form is exactly what you expect here.

On is the 3rd person singular present tense of olla, which means to be.

So:

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

In your sentence:

• Omavastuu on sata euroa.
• literally: The deductible is one hundred euros.

So on simply means is here.

Yes, but Omavastuu on sata euroa is the most neutral and natural order.

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

For example:

• Omavastuu on sata euroa. = neutral statement
• Sata euroa on omavastuu. = possible, but marked and unusual in most contexts
• Omavastuu on 100 euroa. = also completely normal, just using digits

So if you are learning, the safest version to use is:

• Omavastuu on sata euroa.

Yes. Sata is the basic form meaning one hundred.

In this sentence it stays in that basic form:

• sata euroa

But in other structures it can change:

• sadan euron vakuutus = a 100-euro insurance policy
• sadassa eurossa = in one hundred euros

So sata is the basic form, but like other Finnish words, it can be inflected depending on the grammar around it.

Yes, absolutely.

Both are normal:

• Omavastuu on sata euroa.
• Omavastuu on 100 euroa.

Using digits is especially common in practical contexts like:

• insurance documents
• invoices
• websites
• contracts

The grammar stays the same either way:

• 100 euroa works just like sata euroa.