Omaisuus oli kellarissa.

Because oli is the past tense of olla (to be).

• on = is
• oli = was

So:

• Omaisuus on kellarissa. = The property is in the basement.
• Omaisuus oli kellarissa. = The property was in the basement.

Here the subject omaisuus is singular, so the singular past form oli is used.

Kellarissa is in the inessive case, which usually means in or inside something.

The basic noun is:

• kellari = basement / cellar

Add the inessive ending:

• kellari
  • -ssa → kellarissa

So kellarissa means in the basement / in a basement / in the cellar, depending on context.

This is because of vowel harmony, an important Finnish pattern.

Finnish endings often come in two versions:

• -ssa
• -ssä

Words with back vowels like a, o, u usually take the back-vowel ending -ssa.
Since kellari contains a, it takes:

• kellarissa

If a word had only front vowels, it would use -ssä instead.

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

So Finnish often leaves that information to context.
That means kellarissa can mean:

• in the basement
• in a basement

and omaisuus can mean:

• the property
• property
• the assets

The exact English wording depends on the situation, not on a separate article in Finnish.

Because omaisuus is usually treated as a collective or mass noun in Finnish.

Grammatically, it is singular:

• omaisuus oli

even if it refers to a collection of belongings, assets, or property.

This is similar to how English can use a singular word like property or furniture for many individual items.

Yes. It is a very neutral and natural Finnish word order:

• Omaisuus = subject
• oli = verb
• kellarissa = location

So it follows a basic subject + verb + place pattern.

Finnish word order is fairly flexible, though. For example:

• Kellarissa oli omaisuus.

This is also possible, but it gives more emphasis to the location.

A helpful rough breakdown is:

• o-mai-suus
• o-li
• kel-la-ris-sa

A few key points:

• Finnish stress is usually on the first syllable of each word.
• ai in omaisuus is a diphthong.
• uu in -suus is a long vowel.
• ll in kellarissa is a long/doubled consonant.
• ss in kellarissa is also long.

So the doubled letters should be held a bit longer than a single consonant.

These are three common Finnish location forms:

• kellarissa = in the basement (location inside)
• kellariin = into the basement (movement into)
• kellarista = out of the basement / from the basement (movement out)

So:

• Omaisuus oli kellarissa. = The property was in the basement.
• Omaisuus vietiin kellariin. = The property was taken into the basement.
• Omaisuus tuotiin kellarista. = The property was brought from the basement.

This is a very common Finnish pattern with place words.

Here omaisuus is in the nominative singular, which is the basic dictionary form.

In this sentence it functions as the subject:

• omaisuus = subject
• oli = verb

That is why the verb is the singular form oli.

So structurally, the sentence is:

• Omaisuus = subject
• oli = was
• kellarissa = in the basement

Yes. That is the simplest way to make it present tense:

• Omaisuus oli kellarissa. = The property was in the basement.
• Omaisuus on kellarissa. = The property is in the basement.

Nothing else in the sentence needs to change.