Learn Hardcore Finnish: Pesin kädet lämpimällä vedellä ennen iltapalaa. - I washed my hands with warm water before the evening snack.

You can say Minä pesin kädet…, but that usually adds emphasis, like I washed my hands (not someone else).
The neutral everyday version just leaves minä out.

What exactly is the form pesin? Which verb is it from and what tense/person is it?

Pesin is:

the past tense (simple past),
1st person singular (I),
of the verb pestä = to wash.

Conjugation (indicative):

Present: minä pesen – I wash
Past: minä pesin – I washed

So in the sentence, Pesin = I washed.

Why is it kädet (hands, plural) and not the singular käsi (hand)?

In Finnish, you almost always use the plural for body parts that naturally come in pairs:

pesen kädet – I wash (my) hands
harjaan hampaat – I brush (my) teeth
pesen jalat – I wash (my) feet / legs

Using the singular käsi here would sound like you washed only one hand, which is unusual unless you really want to say that specifically.

There’s no my in pesin kädet. Does it still mean I washed my hands?

Yes. With body parts and close personal belongings, Finnish often omits the possessive pronoun when it’s obvious whose body part it is.

Pesin kädet. → I washed my hands.
Pesin hiukset. → I washed my hair.

You can add a possessive if you really want to stress it:

Pesin käteni. – I washed my (own) hands.

But in neutral speech, Pesin kädet is completely natural and is normally understood as my hands.

Why is it kädet and not käsiä? What’s the difference?

This is about object case (total vs. partial).

Pesin kädet.
kädet = total object (formally “accusative”, looks like nominative plural)
→ You washed the hands completely / as a whole action.
Pesin käsiä.
käsiä = partitive plural
→ Suggests an ongoing, incomplete, or partial action, e.g.
- you were in the middle of washing hands,
- or you washed some hands (maybe several people’s hands),
- or you’re describing the activity in general, not the finished result.

In this sentence, you want a finished, complete action before the meal, so kädet is the normal choice.

Could I say Pesin käteni lämpimällä vedellä ennen iltapalaa? What’s the difference from Pesin kädet?

Yes, that sentence is grammatically correct.

Pesin kädet… – neutral, everyday, “I washed my hands.”
Pesin käteni… – adds a clearer sense of “my own hands”, slightly more formal or emphatic.

In many everyday contexts, Finns still prefer Pesin kädet, because the ownership is obvious from the context and the verb form.

Why is lämpimällä vedellä in that form? How does Finnish say with warm water?

Finnish often uses the adessive case (ending -lla / -llä) to express the instrument: with what?

The question word is millä? = with what?

Millä pesit kädet? – With what did you wash your hands?
Lämpimällä vedellä. – With warm water.

So:

lämpimä-llä = warm (adessive singular)
vedellä = water (adessive singular)

Together: lämpimällä vedellä = with warm water (as the washing agent).

Why does lämpimällä also end in -llä? Why not just lämpimä?

Adjectives in Finnish agree with the noun they describe in:

case
number
(and sometimes possession)

Since vedellä is in the adessive singular (because of the “with” meaning), the adjective lämpimä must also be in adessive singular:

nominative: lämmin vesi – warm water
adessive: lämpimällä vedellä – with warm water

So both parts take -llä:

lämpimä + llä
vesi → vede + llä = vedellä

Could I say lämpimässä vedessä or lämmintä vettä instead? How would the meaning change?

Yes, but the meaning changes:

lämpimässä vedessä = in warm water
- Uses inessive case (-ssa / -ssä) → location inside something.
- Pesin kädet lämpimässä vedessä.
  = I washed my hands in warm water
  (e.g. your hands were actually in a bowl or sink full of warm water).
lämmintä vettä = warm water in partitive
- Could appear in other constructions, e.g.
  Käytin lämmintä vettä. – I used warm water.
- But on its own it doesn’t express the “with” instrument idea as clearly as lämpimällä vedellä.

In your sentence, you want “with warm water” as an instrument, so lämpimällä vedellä is the most natural.

Why is it ennen iltapalaa and not ennen iltapala?

The preposition ennen (before) always requires the partitive case:

ennen + partitive

So:

iltapala (nominative) → iltapalaa (partitive)

Examples:

ennen iltapalaa – before (the) evening snack/supper
ennen lounasta – before lunch
ennen kokousta – before the meeting

Using nominative (ennen iltapala) would be incorrect here.

Is the word order fixed? Could I also say Ennen iltapalaa pesin kädet lämpimällä vedellä?

The word order is flexible, and your alternative is fine.

Pesin kädet lämpimällä vedellä ennen iltapalaa.
Neutral; starts with the action (I washed).
Ennen iltapalaa pesin kädet lämpimällä vedellä.
Slightly more emphasis on the time phrase (Before dinner, I washed…).

Both are grammatical and natural. Finnish allows moving time, place, and manner phrases around for emphasis, as long as the verb and its arguments remain clear.