Koira pelasti lapsen hengen.

Here lapsen is the genitive singular of lapsi. The genitive often shows possession, so lapsen hengen means the child's life.

Also, many Finnish nouns change their stem when endings are added. So lapsi uses the stem lapse- in forms like lapsen.

Here hengen is the object form used for a completed, successful action: the dog saved the life completely, not just partially or incompletely. In singular nouns, this total object often looks the same as the genitive form, ending in -n.

Like lapsi, henki also changes stem when inflected: henki → henge- → hengen.

They look the same, but they do different jobs.

• lapsen is genitive and shows possession: the child's
• hengen is the object of the verb: life as the thing being saved

So both have -n, but they are not functioning the same way in the sentence.

In a neutral Finnish sentence, the subject often comes first, and here koira is in the basic nominative form. It is the one doing the action of pelasti.

So the structure is:

Koira = subject
pelasti = verb
lapsen hengen = object phrase

Literally, it is the child's life.

Finnish often expresses possession by putting the possessor in the genitive before the noun:

• lapsen kirja = the child's book
• lapsen henki = the child's life

In this sentence, that whole phrase then becomes the object of the verb.

Henkeä would be the partitive form. Finnish uses the partitive object when the action is incomplete, ongoing, indefinite, or negated.

But here the action is complete and successful: the life was saved. That is why Finnish uses the total object form hengen.

Compare:

• Koira pelasti lapsen hengen. = The dog saved the child's life.
• Koira ei pelastanut lapsen henkeä. = The dog did not save the child's life.

Yes. Finnish word order is fairly flexible, and changing it often changes emphasis rather than basic meaning.

The neutral order is: Koira pelasti lapsen hengen.

But you could also say: Lapsen hengen pelasti koira.

That gives extra emphasis to koira, roughly like It was the dog that saved the child's life.

No. Henki can mean several related things, depending on context, such as life, breath, spirit, or even atmosphere.

In the expression pelastaa jonkun henki, it clearly means save someone's life.

Because Finnish normally says pelastaa jonkun henki for save someone's life. That is the standard expression.

Elämä also means life, but more in the sense of life/existence/a lifetime. In this sentence, henki is the natural word because it refers to someone's life being in danger and then being saved.