Learn Hardcore Finnish: Kuppi oli likainen, joten pesin sen käsin. - The cup was dirty, so I washed it by hand.

Pesin kupin käsin. = I washed the cup by hand.
- Here kupin is the total object in the genitive form, because the washing is completed and happens in the past.

What exactly does joten mean, and how is it different from koska or siksi?

Joten means “so / therefore” and introduces a result or consequence:

Kuppi oli likainen, joten pesin sen käsin.
The cup was dirty, so I washed it by hand.

Compare:

koska = “because” → introduces the reason
- Pesin kupin käsin, koska se oli likainen.
  I washed the cup by hand because it was dirty.
siksi = “for that reason / that’s why” → an adverb, often at the beginning of the result clause
- Kuppi oli likainen, siksi pesin sen käsin.
  The cup was dirty; that’s why I washed it by hand.

So:

joten = so, therefore (linking cause → result)
koska = because (stating the cause)
siksi = for that reason, that’s why (adverb of reason)

What form is pesin, and how is it related to the verb pestä?

Pesin is:

past tense (imperfect)
1st person singular (“I”)
of the verb pestä (“to wash”).

Rough pattern:

Infinitive: pestä = to wash
Present: minä pesen = I wash
Past: minä pesin = I washed

So in the sentence:

pesin = I washed (one person, in the past)

Why is it pesin sen käsin and not pesin se käsin?

The pronoun se changes form when it’s used as an object.

Forms of se (it / that):

Nominative (subject): se
- Se on kuppi. = It is a cup.
Genitive / total object form: sen
- Pesin sen. = I washed it.

In this sentence, sen refers back to kuppi and is the object of pesin:

pesin (I washed) + sen (it) → I washed it

You could also say:

Pesin kupin käsin. = I washed the cup by hand.

There kupin (genitive) is the full object instead of the pronoun.

Why sen and not sitä? What’s the difference?

Both sen and sitä are object forms of se, but they’re used in different situations:

sen = total object (often completed action, countable whole)
sitä = partitive object (often incomplete action, “some of it”, ongoing, or with certain verbs)

Here the washing is a completed action on the whole cup, so total object sen is natural:

Pesin sen käsin. = I washed it by hand. (washed the whole cup)

Examples to contrast:

Join sen. = I drank it (all of it).
Join sitä. = I drank some of it / I was drinking it. (not necessarily all)

What does käsin literally mean, and why does it mean “by hand”?

Käsin comes from käsi = hand.

Käsin is an old instructive/adverbial form that means “with (the) hand(s), by hand”. It’s used as a fixed expression:

tehdä käsin = to do (something) by hand
pestä käsin = to wash by hand

It does not mean “into the hands” or “to the hands” (that would be käsiin, with a double i and a different case).

So:

pesin sen käsin ≈ “I washed it using (my) hands” → I washed it by hand.

Why doesn’t Finnish say “by my hand(s)” like in English “with my hands”?

Finnish often leaves out possessive pronouns where English would use my / your / his if the owner is obvious from context.

Because I am doing the washing:

Pesin sen käsin.
Literally: I washed it (with) hands.
Naturally understood as my hands.

You usually don’t need to say minun käsilläni (with my hands) unless you specifically want to emphasize whose hands are involved.

Could you say Kuppi oli likaisena instead of Kuppi oli likainen? What’s the difference?

Both are possible, but they don’t mean exactly the same thing.

Kuppi oli likainen.
- Normal, neutral way to say: The cup was dirty.
- likainen is a predicate adjective in the nominative.
Kuppi oli likaisena.
- Uses essive case (-na/-nä), often meaning “in a dirty state / while dirty”.
- Focuses more on the state or role at a certain time: The cup was in a dirty condition (often as background to something else happening).

In your example sentence, the neutral, most common option is:

Kuppi oli likainen, joten pesin sen käsin.

How is likainen working grammatically here? Does it have to agree with kuppi?

Yes, likainen is an adjective that describes kuppi.

In this structure with olla (“to be”):

Subject: Kuppi (nominative singular)
Predicate adjective: likainen (also nominative singular)

So:

Kuppi oli likainen.
Kupit olivat likaisia. (The cups were dirty.)

When the subject is plural, the adjective also appears in plural (likaisia, partitive plural in this predicative position), but in this simple singular case you just see likainen.

Can I change the word order, for example: Pesin sen käsin, koska kuppi oli likainen?

Yes, that’s a perfectly natural sentence, just with slightly different linking:

Kuppi oli likainen, joten pesin sen käsin.
- Emphasis: dirty cup → result: I washed it.
Pesin sen käsin, koska kuppi oli likainen.
- Emphasis: I washed it by hand → reason: because the cup was dirty.

Both are correct Finnish.
Common variants:

Kuppi oli likainen, siksi pesin sen käsin.
Kuppi oli likainen. Pesin sen käsin. (Two separate sentences; the causal link is clear from context.)

Finnish word order is relatively flexible, but connectors like joten / koska help show the logical relationship between the clauses.

How do I know when kuppi means “a cup” vs “the cup”?

Finnish has no articles (a, an, the), so kuppi can mean either “a cup” or “the cup”. The meaning is decided by context.

Ostin kupin. = I bought a cup. (usually new information)
Kuppi oli likainen, joten pesin sen käsin.
- In English you’d probably say: The cup was dirty, so I washed it by hand.
- It’s a specific cup that both speaker and listener can identify from context.

So you choose a or the in English when translating, but in Finnish the word kuppi itself doesn’t change.