Learn Hardcore Finnish: Pesen astiat käsin, koska astianpesukone on rikki. - I wash the dishes by hand because the dishwasher is broken.

Pesen is the 1st person singular present tense form of the verb pestä (to wash).
Finnish marks the subject on the verb, so you usually don’t need minä (I) unless you want emphasis:

(Minä) pesen. = I wash / I am washing.

Why is astiat in that form? Why not astioita?

Astiat is the plural total object (“the dishes” as a complete set you’re washing). In Finnish, objects often split into:

Total object (you wash them up completely) → astiat
Partitive object (some/unspecified amount or ongoing without focusing on completion) → astioita

So:

Pesen astiat. = I wash (all) the dishes / wash the dishes (to completion).
Pesen astioita. = I wash some dishes / I’m doing dishes (focus on process or an unspecified amount).

How can astiat be the object if it looks like a normal plural nominative?

In Finnish, the plural total object often uses the same form as plural nominative. That’s normal:

singular total object often shows as genitive (e.g., pesen astian = I wash the dish)
plural total object often shows as nominative plural (pesen astiat)

So astiat here functions as an object even though it “looks nominative.”

What does käsin mean grammatically, and why isn’t it kädellä?

Käsin means by hand / manually and is in the instructive case, which is used in a few common adverb-like expressions (especially for “by means of”):

käsin = by hand
jalan = on foot
postitse = by mail (not instructive, but similar “manner” idea)

You can say kädellä, but it usually means more literally with (a) hand and often sounds odd for this general “manually” meaning. The idiomatic choice is käsin.

Why is there a comma before koska?

In Finnish, a comma is typically used before a subordinate clause introduced by koska (because):

Pesen astiat käsin, koska …
This matches standard Finnish punctuation rules.

Does koska always mean “because”? I’ve seen it mean “when/since.”

Koska can mean:

because (reason)
sometimes since in the “because” sense

For “when” (time), Finnish more commonly uses kun:

Kun tulin kotiin, … = When I came home, …

In your sentence, koska is clearly the “because” meaning.

Why is the word order koska astianpesukone on rikki and not something like “because is the dishwasher broken”?

Finnish doesn’t form questions by inversion the way English does. In a normal statement clause, the verb usually stays in the same place:

astianpesukone on rikki = the dishwasher is broken

A yes/no question would typically use the question clitic -ko/-kö:

Onko astianpesukone rikki? = Is the dishwasher broken?

So the koska-clause keeps normal statement order.

What is astianpesukone made of? Why is there astian- at the start?

Astianpesukone is a compound word:

astia = dish, container
astian = genitive singular (“of a dish / dish-”) used as the first part in compounds
pesu = washing
kone = machine

So literally it’s something like dish-washing machine → dishwasher. The genitive astian- is a very common way to build compounds in Finnish.

Why is it on rikki? What part of speech is rikki?

Rikki means broken and is used in a fixed “state” expression with olla (to be):

olla rikki = to be broken / to be out of order

Even though English uses an adjective (“broken”), Finnish often uses these state words like rikki after olla. You’ll see similar patterns:

on valmis = is ready
on auki = is open
on kiinni = is closed

Is rikki the same as rikkinäinen?

They’re related but not identical:

on rikki = is broken / not working (very common, idiomatic)
rikkinäinen = “broken/damaged” as a descriptive adjective, often about a physical object being in a damaged condition

Often both can work, but on rikki is the default for appliances:

Astianpesukone on rikki. (very natural)
Astianpesukone on rikkinäinen. (possible, but less idiomatic)

Why isn’t there a word for “the” in Finnish?

Finnish has no articles (“a/the”). Definiteness is usually understood from context or shown with other means (word order, pronouns, cases, etc.).
So astiat can mean the dishes in context without any “the.”

Could I add “my” (my dishwasher) or “the dishwasher” more explicitly?

Yes:

… koska astianpesukoneeni on rikki. = because my dishwasher is broken
(-ni is the 1st person singular possessive suffix)
… koska tämä astianpesukone on rikki. = because this dishwasher is broken

Most of the time, leaving it as astianpesukone is fine if it’s obvious which one you mean.

Is this sentence in the “I’m washing (right now)” sense or “I wash (in general)” sense?

Finnish present tense covers both:

Pesen astiat… can mean I’m washing the dishes (now) or I wash the dishes (generally).

Context usually makes it clear. With koska astianpesukone on rikki, it often implies a current situation (“so I’m washing by hand”).