Lapsi löytää leluauton sohvan alta.

Because it is the object of the verb löytää (to find), and in this sentence it is a total object: the action has a clear result, and the toy car is found as a whole.

So:

• leluauto = toy car
• leluauton = object form used here

For learners, this often looks like the genitive singular ending -n, and in many sentences that is exactly what you will see with a singular total object.

Here sohvan is not the object. It is in the genitive because it comes before the postposition alta.

In Finnish, many postpositions work like this:

• noun in genitive
• then the postposition

So:

• sohva = sofa
• sohvan alta = from under the sofa

This is a very common structure in Finnish.

Because alta expresses movement from under something, while alla expresses location under something.

Compare:

• sohvan alla = under the sofa / beneath the sofa
• sohvan alta = from under the sofa

In this sentence, the toy car is found from under the sofa, so alta is the right choice.

Lapsi is the subject of the sentence, and it is in the nominative singular, which is the basic dictionary form here.

• lapsi = child

In a normal active sentence, the subject is usually in the nominative:

• Lapsi löytää... = The child finds...

So lapsi stays in its basic form because it is the one doing the action.

Löytää is 3rd person singular present tense.

It matches the singular subject lapsi:

• minä löydän = I find
• sinä löydät = you find
• hän löytää = he/she finds
• lapsi löytää = the child finds

So this sentence is in the present tense and means something like the child finds.

Finnish has no articles, so there is no direct equivalent of a or the.

That means:

• lapsi can mean a child or the child
• leluauton can mean a toy car or the toy car
• sohvan can mean a sofa’s / the sofa’s, depending on the structure

The exact meaning comes from context, not from articles.

Yes, leluauto is a compound noun, and Finnish uses compounds very often.

It is made of:

• lelu = toy
• auto = car

Together:

• leluauto = toy car

When a compound is inflected, the case ending usually goes on the last part:

• leluauto
• leluauton

This is very normal in Finnish.

This is a very natural, neutral word order in Finnish:

• Lapsi = subject
• löytää = verb
• leluauton = object
• sohvan alta = place/source expression

So the basic structure is close to English:

subject + verb + object + adverbial

Finnish word order is fairly flexible, though, because the case endings show the roles of the words. You could also say:

• Lapsi löytää sohvan alta leluauton.

That version is also grammatical, but it puts slightly different emphasis on the location.

Yes. Sohvan alta works as a postpositional phrase, and you should think of it as one unit meaning from under the sofa.

It has two parts:

• sohvan = noun in the genitive
• alta = postposition

Together they answer the question from where?

So in the sentence:

• Lapsi löytää leluauton = the child finds the toy car
• sohvan alta = from under the sofa

No, not in standard Finnish.

With alta, you normally need the noun in the genitive:

• sohvan alta = correct
• sohva alta = incorrect

This is something you simply have to learn with postpositions: they often require a specific case, and alta takes the genitive form before it.

Very often, yes, when the thing found is seen as a complete result.

Here, the child finds the toy car as a whole, so the singular total object appears as:

• leluauton

If the object were plural, or if the sentence were negative, the form could change. For example, in negative sentences Finnish often uses the partitive:

• Lapsi ei löydä leluautoa. = The child does not find the toy car / a toy car.

So the object form in Finnish depends a lot on how the action is viewed.

The key is the ending on the postposition:

• alla = under / at a location under
• alta = from under
• alle = to under / to a position under

This is a very common Finnish pattern for location and movement:

• missä? = where? → alla
• mistä? = from where? → alta
• mihin? = to where? → alle

So sohvan alta clearly shows movement/source, not just location.