Käytämme esiliinaa, kun vaivaamme savipaakkua ja tasoitamme epätasaista pintaa.

-mme is the personal ending for first person plural in the present tense, so it means we.

• käytämme = we use / we wear
• vaivaamme = we knead
• tasoitamme = we smooth / level

Finnish often puts the subject information directly on the verb, instead of always using a separate pronoun.

Because the verb ending already tells you the subject.

In Finnish, käytämme already means we use, so me is not necessary. You can add me if you want extra emphasis or contrast:

• Käytämme esiliinaa. = neutral, normal
• Me käytämme esiliinaa. = we use an apron (maybe contrasting with someone else)

This omission of subject pronouns is very common in Finnish.

Finnish has no articles. There is no direct equivalent of English a, an, or the.

So Finnish simply says:

• esiliinaa not an apron / the apron
• savipaakkua not a lump of clay / the lump of clay

Whether something is definite or indefinite is understood from context.

Here esiliinaa is in the partitive singular.

With käyttää, the object is often in the partitive when the meaning is an ongoing or habitual activity, especially with clothing or things being used:

• käyttää esiliinaa = to wear/use an apron
• käyttää silmälaseja = to wear glasses

So Käytämme esiliinaa presents apron-wearing as part of the activity, not as a completed action affecting a whole object.

For the same general reason: the sentence describes an ongoing process, not a clearly completed result.

• vaivaamme savipaakkua = we are kneading a lump of clay
• tasoitamme epätasaista pintaa = we are smoothing an uneven surface

The partitive object is very common when the action is seen as:

• ongoing
• incomplete
• repeated/habitual
• focused on the process rather than the finished result

A useful contrast:

• tasoitamme epätasaista pintaa = we are smoothing an uneven surface
• tasoitamme pinnan = we smooth the surface completely / make it smooth

So the partitive here fits the idea of work in progress.

Because in Finnish, adjectives usually agree with the noun in case and number.

The basic form is:

• epätasainen pinta = an uneven surface

But here the noun is in the partitive singular:

• pintaa

So the adjective must also be in the partitive singular:

• epätasaista pintaa

This kind of agreement is a very important Finnish pattern.

kun here means when.

It introduces a subordinate clause telling when the action in the main clause happens:

• Käytämme esiliinaa = main clause
• kun vaivaamme savipaakkua ja tasoitamme epätasaista pintaa = when-clause

So the structure is basically:

• We use an apron, when we knead... and smooth...

In natural English you may or may not keep the comma, but in Finnish the subordinate clause structure is very straightforward.

Because Finnish normally uses a comma to separate a main clause from a subordinate clause.

So:

• Käytämme esiliinaa, kun...

is standard punctuation.

This is one place where Finnish comma use is often more regular than English.

Yes, all three verbs are in the present tense:

• käytämme
• vaivaamme
• tasoitamme

In Finnish, the present tense is often used not only for something happening right now, but also for:

• habitual actions
• general truths
• instructions
• descriptions of a process

So this sentence sounds natural as a general statement about what we do during this kind of work.

Because Finnish verbs change stem when they are conjugated.

For example:

• käyttää → käytä-
  • -mme = käytämme
• tasoittaa → tasoita-
  • -mme = tasoitamme
• vaivata → vaivaa-
  • -mme = vaivaamme

In käyttää and tasoittaa, the double tt of the infinitive becomes a single t in these forms. This is part of a common Finnish pattern often called consonant gradation.

So the dictionary form is not always the same as the stem used in personal forms.

Yes. savipaakku is a compound:

• savi = clay
• paakku = lump / clump

In Finnish compounds, usually only the last part gets the case ending:

• savipaakku = lump of clay
• savipaakkua = partitive singular of savipaakku

This is a very common Finnish pattern. The whole compound acts like one word, and the grammatical ending is added at the end.