Teidän autonne on iso.

Because standard Finnish often marks possession in two places:

• with a possessive pronoun: teidän = your
• with a possessive suffix on the noun: autonne = your car

So teidän autonne literally looks a bit like your car-your, but this is normal in standard Finnish.

This pattern is especially common in careful written Finnish and in teaching materials. In everyday spoken Finnish, people often simplify it and may leave out the suffix, for example:

• teidän auto on iso

But in standard Finnish, Teidän autonne on iso is a very good model sentence.

Teidän is the genitive form of te, which means you in the plural.

So teidän means:

• your when talking to more than one person
• also sometimes your when speaking politely/formally to one person

That means Teidän autonne can mean either:

• your car (belonging to several people), or
• your car (belonging to one person addressed formally)

You can only tell which meaning is intended from the situation.

Autonne comes from:

• auto = car
• -nne = possessive suffix meaning your (plural/formal)

So:

• auto → autonne = your car

The final result is not auto-nne written separately; it becomes one word: autonne.

The possessive suffix for your (plural/formal) is -nne.

When it attaches to auto, you get:

• auto
  • nne → autonne

Finnish often joins endings directly to the noun stem, and this is the normal form here.

There is no extra case ending in this sentence on auto, because the noun is just the subject of the sentence in its basic nominative form, plus the possessive suffix.

It is talking about one car.

That is because auto is singular. If it were plural, the noun would look different.

So:

• autonne = your car not
• your cars

Even though teidän can refer to more than one person, the thing owned here is still singular.

Because the verb agrees with the subject, and the subject here is autonne = your car, which is singular.

So:

• autonne = one car
• therefore on = is

The sentence is really structured as:

• Your car is big

not

• You are big

So the number of the owners does not control the verb here; the number of the car does.

Both iso and suuri can mean big/large.

In many everyday situations:

• iso is the more common, basic, conversational word
• suuri can sound a little more formal, abstract, or literary in some contexts

So Teidän autonne on iso is a very natural beginner sentence meaning Your car is big.

No. Here iso stays in its basic form because it describes a singular subject in a simple sentence:

• Teidän autonne on iso. = Your car is big.

Finnish adjectives do change in many other situations, especially in cases like:

• isossa autossa = in a big car

But after on in a basic sentence like this, the adjective usually appears in its basic dictionary form.

Yes. Autonne on iso is also correct and means Your car is big.

Because -nne already shows your, the pronoun teidän is not always necessary.

So you may see both:

• Autonne on iso.
• Teidän autonne on iso.

Using both can make the possession extra clear or sound more formal/complete.

In everyday spoken Finnish, many speakers do say things like that, especially in casual speech.

However, in standard written Finnish, the expected form is:

• teidän autonne on iso

So for learning good standard grammar, it is best to use the version with the possessive suffix.

A simple way to remember it:

• spoken Finnish often drops the possessive suffix
• standard Finnish usually keeps it

A careful approximate pronunciation is:

• tei-dän

Some helpful points:

• ei is a diphthong, said together in one syllable
• ä is a front vowel, somewhat like the vowel in British cat for many speakers, though not exactly the same
• stress in Finnish is usually on the first syllable: TEI-dän

A careful approximate pronunciation is:

• au-ton-ne

Helpful details:

• au is a diphthong, a bit like the vowel movement in cow
• the nn is a long consonant, so the n is held slightly longer than a single n
• stress is on the first syllable: AU-ton-ne

That long nn matters in Finnish, because consonant length can change meaning.

The normal neutral word order is:

• Teidän autonne on iso.

But Finnish word order is more flexible than English, because endings show grammatical relationships.

For example, you could also say:

• Autonne on iso.

That still means Your car is big.

Different word orders can change the emphasis, but the version you were given is the most straightforward neutral one.

Normally, teidän is written with a lowercase t in the middle of a sentence.

If you see Teidän with a capital letter, it may be:

• because it starts the sentence, or
• sometimes because of a polite style, especially in formal writing

In your sentence, it is capitalized simply because it is the first word of the sentence.