Breakdown of Valottaessasi kuvaa pimiön täytyy olla täysin pimeä.
olla
to be
kuva
picture
täytyä
to have to
pimeä
dark
Questions & Answers about Valottaessasi kuvaa pimiön täytyy olla täysin pimeä.
What form is valottaessasi, and how does it mean 'when you expose'?
This is a temporal construction (often called the second infinitive inessive). It is formed using the verb valottaa (to expose) with the -essa ending to mean 'while/when exposing', plus the possessive suffix -si to specify 'when you expose'. It is a more advanced, compact way of saying kun valotat.
Since this lesson is about vasta, how would I say the darkroom must be dark only when (or not until) you expose the picture?
You can place the adverb vasta directly before the temporal structure: Vasta valottaessasi kuvaa pimiön täytyy olla täysin pimeä. Remember that while English often uses a negative phrase like 'not until', Finnish uses the affirmative vasta to express this kind of delayed timing.
Why is kuvaa in the partitive case here?
Because the temporal construction valottaessasi focuses on the ongoing, simultaneous action of exposing the picture ('while you are exposing'). Since the action is viewed as a continuous process rather than a completed whole, its object takes the partitive case (kuvaa).
Why does the word for darkroom (pimiö) take the genitive ending (pimiön)?