Tyttö haluaa jäätelöä, mutta minä otan kahvin.

Finnish does not have articles. That means tyttö can mean a girl or the girl, and kahvin can mean a coffee or the coffee, depending on context.

English has to choose an article, but Finnish usually leaves that information to context.

Tyttö is the subject of the first clause, so it is in the nominative, which is the basic dictionary form.

In Tyttö haluaa jäätelöä, the girl is the one doing the wanting, so tyttö stays in its basic form.

Haluaa is the third person singular present tense of haluta, meaning to want.

So:

• minä haluan = I want
• sinä haluat = you want
• hän haluaa = he/she wants

With tyttö, haluaa means the girl wants.

Jäätelöä is the partitive singular of jäätelö.

Finnish often uses the partitive when the object is:

• an unspecified amount
• a mass noun
• something not presented as a complete, bounded item

Here, jäätelöä is like saying some ice cream. That is why the sentence uses the partitive.

Yes, in some contexts, but the meaning changes a bit.

Jäätelöä is the natural choice when the speaker means some ice cream or ice cream in a general sense.
Jäätelön would suggest a more specific, complete object, such as one particular ice cream or one whole serving.

With haluta, the partitive is very common when the thing wanted is not very specifically defined.

Yes. Otan already shows that the subject is I, so minä is not grammatically necessary.

It is included here for emphasis or contrast. Because the sentence has mutta meaning but, minä helps underline the contrast:

• Tyttö haluaa jäätelöä, mutta minä otan kahvin.
• The girl wants ice cream, but I’ll have coffee.

Without minä, the sentence would still be correct:

• Tyttö haluaa jäätelöä, mutta otan kahvin.

Otan is the first person singular present tense of ottaa, meaning to take.

Finnish often uses the present tense for near-future actions, decisions, and orders. So in a context like choosing food or drink, otan kahvin naturally means:

• I’ll have coffee
• I’ll take coffee

This is very normal Finnish usage.

Kahvin is the form used for a total object in the singular. With singular nouns, that form usually looks the same as the genitive ending -n.

Here it suggests a complete, chosen item: a coffee, the coffee, or one coffee serving.

Compare:

• Otan kahvin = I’ll have a coffee / I’ll take the coffee
• Otan kahvia = I’ll have some coffee

So kahvin feels more like a whole item, while kahvia feels more like an unspecified amount of coffee.

In Finnish, a comma is normally used before mutta when it joins two clauses that each have their own verb.

Here the two clauses are:

• Tyttö haluaa jäätelöä
• minä otan kahvin

Since both are full clauses, the comma is expected.

This sentence uses a very normal subject-verb-object order:

• Tyttö haluaa jäätelöä
• minä otan kahvin

Finnish word order is fairly flexible, though, because case endings and verb endings carry a lot of grammatical information. You can change the order for emphasis, but the neutral version is the one in the sentence.

A few sounds here are unfamiliar to many English speakers:

• y has no exact English equivalent; it is like ee said with rounded lips
• ä is roughly like the a in cat
• ö also has no exact English equivalent; it is similar to German ö or French eu

Also, double letters matter in Finnish:

• tt in tyttö is held longer than a single t
• ää in jäätelöä is a long vowel

A rough breakdown is:

• tyt-tö
• jää-te-lö-ä

Length is important in Finnish, so double vowels and double consonants should be clearly longer than single ones.