Yksi sormi on kipeä, joten kirjoitan hitaasti.

Because yksi behaves differently from larger numbers in Finnish.

• yksi sormi = one finger
• kaksi sormea = two fingers

With yksi, the noun is normally in the singular nominative:

• yksi talo = one house
• yksi päivä = one day

With numbers 2 and up, the noun is usually in the partitive singular:

• kaksi taloa
• kolme päivää

So yksi sormi is exactly what you would expect.

Sormi is in the nominative singular.

In the clause Yksi sormi on kipeä, the phrase yksi sormi is the subject, and subjects are very often in the nominative.

So the structure is:

• yksi sormi = subject
• on = verb
• kipeä = predicate adjective

Because kipeä is a predicate adjective after the verb on.

In Finnish, when an adjective describes the subject through a verb like olla (to be), it is usually in the nominative singular if the subject is singular.

So:

• Sormi on kipeä. = The finger is sore.
• Talo on suuri. = The house is big.
• Kirja on uusi. = The book is new.

You would use forms like kipeässä or kipeän only in different grammatical roles, not here.

Here kipeä means something like sore, painful, or hurting.

It is a common Finnish adjective used for body parts and also for a person who feels unwell.

For example:

• Sormi on kipeä. = My finger hurts / The finger is sore.
• Jalka on kipeä. = My leg hurts / The leg is sore.
• Olen kipeä. = I’m sick / I’m not well.

So the exact English translation depends on context. In this sentence, with sormi, it means the finger is painful or sore.

Finnish often leaves possession unstated when it is obvious from context.

So Yksi sormi on kipeä can naturally mean:

• One finger is sore
• One of my fingers hurts

Even without my, the sentence is normal if the speaker is talking about their own situation.

If you wanted to make it more explicit, Finnish has ways to do that, but in everyday language it is very common to leave it understood.

Because Finnish verb endings usually already show the subject.

Kirjoitan means I write or I am writing. The ending -n marks first person singular.

So:

• kirjoitan = I write
• kirjoitat = you write
• kirjoittaa = he/she writes
• kirjoitamme = we write
• kirjoitatte = you all / you formal write
• kirjoittavat = they write

Because the verb form already tells you the subject, minä is often omitted unless you want emphasis or contrast.

Because Finnish dictionaries list verbs in the infinitive, but sentences use a conjugated form.

The basic form is:

• kirjoittaa = to write

But here the speaker is saying I write / I am writing, so the verb must be conjugated:

• (minä) kirjoitan = I write / I am writing

So kirjoittaa is the base form, and kirjoitan is the form that fits this sentence.

It can mean either one, depending on context.

Finnish present tense often covers both:

• I write
• I am writing

So kirjoitan hitaasti can mean:

• I write slowly
• I am writing slowly

In this sentence, because of the situation with the sore finger, English would often prefer I am writing slowly, but Finnish uses the same present-tense form.

Because hitaasti is an adverb, while hidas is an adjective.

• hidas = slow
• hitaasti = slowly

In the sentence, the word describes how the writing happens, so Finnish needs an adverb:

• kirjoitan hitaasti = I write slowly

A very common way to form adverbs in Finnish is to add -sti:

• hidas → hitaasti
• varma → varmasti
• kaunis → kauniisti

Joten means something like so, therefore, or that’s why.

It links the first clause to the result in the second clause:

• Yksi sormi on kipeä = One finger is sore
• joten kirjoitan hitaasti = so I write slowly

It shows that the second part follows as a consequence of the first part.

This is a very common question.

• koska usually means because
• joten usually means so / therefore

Compare:

• Kirjoitan hitaasti, koska yksi sormi on kipeä.
  = I write slowly because one finger is sore.

• Yksi sormi on kipeä, joten kirjoitan hitaasti.
  = One finger is sore, so I write slowly.

Both sentences connect the same ideas, but from different directions.

Because joten connects two clauses, and in Finnish this is normally written with a comma.

The two clauses are:

1. Yksi sormi on kipeä
2. joten kirjoitan hitaasti

So the comma helps mark the boundary between them.

This is standard Finnish punctuation.

The word order is natural and neutral, but Finnish word order is more flexible than English word order.

This sentence has a straightforward structure:

• Yksi sormi on kipeä, joten kirjoitan hitaasti.

That is the most neutral way to say it.

Because Finnish uses endings to show grammatical roles, words can sometimes be moved for emphasis. But for a learner, this version is the safest and most standard one to use.

Yes. That would also be natural, but the meaning changes slightly.

• Sormi on kipeä. = A finger is sore / My finger hurts.
• Yksi sormi on kipeä. = One finger is sore.

Adding yksi emphasizes that it is specifically one finger, perhaps not all of them.

So yksi is not required for grammar, but it adds information.

It could be, but joten is simply a more compact and natural connector here.

• Yksi sormi on kipeä, joten kirjoitan hitaasti.
  = One finger is sore, so I write slowly.

A longer alternative like sen takia or siksi is possible in some contexts, but joten is a very normal way to express consequence in one sentence.

So the sentence is concise and idiomatic as it stands.