Onnettomuus tappoi kaksi ihmistä.

They are:

• onnettomuus = accident
• tappaa = to kill
• kaksi = two
• ihminen = person, human being

So tappoi and ihmistä are inflected forms, not dictionary forms.

Tappoi is the 3rd person singular past tense of tappaa.

So:

• tappaa = to kill / kills (depending on context)
• tappoi = killed

It is 3rd person singular because the subject, onnettomuus, is singular.

Because it is the subject of the sentence.

In a normal active Finnish sentence, the subject is usually in the nominative form, which is the basic dictionary form for many nouns. Here:

• onnettomuus = the thing doing the action
• tappoi = the action
• kaksi ihmistä = the thing affected by the action

So onnettomuus stays in the nominative.

Because after numbers greater than one, Finnish normally uses the noun in the singular partitive.

So:

• yksi ihminen = one person
• kaksi ihmistä = two people
• kolme ihmistä = three people

This is very different from English. English uses a normal plural after numbers, but Finnish usually uses:

• the number
• • a singular partitive noun

That is why ihmistä is correct here.

Because ihminen is one of the common Finnish noun types ending in -nen.

With these nouns, the stem changes in other forms. Very roughly:

• ihminen → base form
• ihmis- / ihmise- → stem used in other cases
• ihmistä → partitive singular

So this is not a random change. It follows a common pattern:

• nainen → naista
• ihminen → ihmistä
• hevonen → hevosta

Learners usually just need to get used to the fact that -nen words change stem.

Because this sentence uses a total object, not a partitive object.

The meaning is that the accident killed two people completely as a finished result. In that kind of sentence, a numeral phrase usually appears as:

• kaksi ihmistä

not

• kahta ihmistä

A useful contrast is negation:

• Onnettomuus tappoi kaksi ihmistä. = The accident killed two people.
• Onnettomuus ei tappanut kahta ihmistä. = The accident did not kill two people.

So the noun ihmistä is there because numerals require it, but the form of the numeral itself can still change depending on the grammar of the sentence.

No, not here.

This is an important point for English speakers:

• ihmistä is in the partitive because it follows the number kaksi
• that does not mean the killing was incomplete

In fact, the sentence describes a completed event in the past. The idea is a whole, finished result: two people were killed.

So here the partitive on ihmistä comes from the number construction, not from incomplete action.

Because Finnish has no articles.

So onnettomuus can mean:

• an accident
• the accident

The exact meaning depends on context.

Likewise, kaksi ihmistä simply means two people. Finnish does not need articles the way English does.

Yes. This is the most neutral basic order:

• Subject – Verb – Object
• Onnettomuus tappoi kaksi ihmistä.

Finnish word order is more flexible than English word order, but this version is the standard, neutral one.

Other orders are possible for emphasis or style, but for a learner, this sentence is the safest model to remember.

Usually no. Tappoi is the simple past: killed.

If you want has killed, Finnish normally uses the perfect:

• Onnettomuus on tappanut kaksi ihmistä.

So:

• tappoi = killed
• on tappanut = has killed

In Finnish, double letters are pronounced as longer sounds, not just written decoration.

Examples here:

• onnettomuus has double tt and long uu
• tappoi has double pp

Also, Finnish stress usually falls on the first syllable of each word:

• ON-net-to-muus
• TAP-poi
• KAK-si
• IH-mis-tä

Length matters in Finnish, so it is worth paying attention to those double letters.