Pieni tyttö alkaa itkeä, kun hänen jäätelönsä putoaa maahan.

Because pieni tyttö is the subject of the sentence, so both words are in the basic dictionary form, the nominative.

• pieni = small, little
• tyttö = girl

So pieni tyttö means a small/little girl or the little girl, depending on context.

If it were pienen tytön, that would be a different case, for example showing possession or being the object of some expressions.

alkaa means to begin / to start, and itkeä means to cry.

Together, alkaa itkeä means starts to cry or begins crying.

This is a very common Finnish structure:

• alkaa + infinitive

For example:

• Alan lukea. = I start reading.
• Lapsi alkaa nauraa. = The child starts to laugh.

After alkaa, Finnish normally uses the first infinitive, which is the basic dictionary form of the verb.

So:

• alkaa itkeä = starts to cry
• alkaa puhua = starts to speak
• alkaa sataa = it starts to rain

Even though English often uses to + verb or -ing, Finnish just uses the infinitive form directly here.

kun here means when.

It introduces a time clause:

• kun hänen jäätelönsä putoaa maahan
• when her ice cream falls to the ground

So the whole sentence means that the crying begins at the moment that the ice cream falls.

In other contexts, kun can sometimes also mean as or even because, depending on usage, but here when is the natural meaning.

Yes, this is something English speakers often notice.

• hänen = his / her
• jäätelö = ice cream
• jäätelönsä = his/her ice cream, with a possessive suffix

So hänen jäätelönsä literally contains possession twice from an English point of view:

• once in the pronoun hänen
• once in the suffix -nsä

This is normal Finnish.

The suffix -nsä means his/her/their.
So jäätelönsä by itself can already mean his/her ice cream.

Finnish often uses both:

• hänen jäätelönsä

This is especially common in standard written Finnish.

Yes, often you can.

• hänen jäätelönsä = her/his ice cream
• jäätelönsä = her/his/their ice cream

The version with hänen is clearer and more explicit.
Without hänen, the possessive is still there because of -nsä, but the owner may depend more on context.

In standard Finnish, both are possible, but hänen jäätelönsä is very natural when you want to be clear.

Because maahan is a case form meaning onto the ground / to the ground.

The basic word is:

• maa = ground, land, earth

The form maahan is the illative case, which often expresses movement into or onto something.

So:

• putoaa maahan = falls to the ground

Some similar examples:

• menen kouluun = I go to school
• tulee huoneeseen = comes into the room

Here the idea is movement toward a place, not just the place itself.

Finnish often uses the present tense in sentences that describe a general situation, a story, or a vivid event.

• putoaa = falls
• alkaa = starts

So the sentence can mean something like:

• The little girl starts to cry when her ice cream falls to the ground.

This works well as a general statement or a narrative present.

If you wanted a clearly past event, you could say:

• Pieni tyttö alkoi itkeä, kun hänen jäätelönsä putosi maahan.
• The little girl started to cry when her ice cream fell to the ground.

This is the most neutral Finnish word order.

Main clause:

• Pieni tyttö alkaa itkeä
  The little girl starts to cry

Subordinate clause:

• kun hänen jäätelönsä putoaa maahan
  when her ice cream falls to the ground

Finnish word order is fairly flexible, but this order is natural because it presents:

1. the subject
2. the main action
3. the time clause explaining when it happens

You could move things around for emphasis, but this version is the plain, standard one.

Yes. jäätelö means ice cream, and in context it can refer to the ice cream the girl is holding, such as an ice cream cone or scoop.

Finnish often leaves that kind of detail unstated if the meaning is obvious from context.

So:

• hänen jäätelönsä putoaa maahan naturally means
• her ice cream falls to the ground

Even if English speakers might imagine her ice cream cone more specifically, Finnish does not need to spell that out unless it matters.

The ending -nsä is a third-person possessive suffix.

It means:

• his
• her
• their

So:

• jäätelö = ice cream
• jäätelönsä = his/her/their ice cream

This suffix is attached to the noun and marks possession directly on it.

Some more examples:

• talonsa = his/her/their house
• kirjansa = his/her/their book
• äitinsä = his/her/their mother

In this sentence, hänen makes it specifically his/her, not their, because hänen is singular.

Usually, yes. pieni means small or little.

In pieni tyttö, it means little girl in a neutral descriptive sense.

English little can sometimes sound affectionate, emotional, or even slightly patronizing depending on context. Finnish pieni is usually more straightforward and literal, though it can also sound warm depending on the situation.

So pieni tyttö is a very natural way to say little girl.