In Finnish, the clitics -kin and -kaan / -kään, and some words (mainly adverbs or conjunctions) in which they have become lexicalized, form polarity pairs, where the former is used as a positive polarity item and the latter as a negative polarity item.
This means that in affirmative and negative sentences, the positive (-kin) and negative (-kaan) polarity forms are generally used, respectively:
Kesä ehkä sittenkin meni jo. (positive)
- Perhaps summer is already gone, after all.
Ei tämä sittenkään toimi. (negative)
- This won't work after all.
However, in interrogative sentences (questions), either can be used, but the nuance is different:
Oliko tämä sittenkin hyvä idea? (positive)
- Was this a good idea after all?
Oliko tämä sittenkään hyvä idea? (negative)
- Was this a good idea after all?
Note that the polarity also applies to certain grammatical features, such as the marking of telic objects in sentences:
Onko hän kuitenkin syönyt kakun? (positive)
- Did he eat the cake anyway?
Onko hän kuitenkaan syönyt kakkua? (negative)
- Did he eat the cake anyway?
The polarity of these words does not have to correlate with the presence or absence of ei in the sentence:
Eikö tämä sittenkin ollut hyvä idea? (positive)
- Wasn't this a good idea after all?
Eikö tämä sittenkään ollut hyvä idea? (negative)
- Wasn't this a good idea after all?
Like in English, there are environments where negative polarity items can appear in affirmative sentences and vice versa (some examples):
Hänellä tuskin on harmaintakaan aavistusta.
- He barely has any idea.
Vain harva enää uskoo, että ongelmaa voidaan ratkaista millään keinolla.
- Only few now think there is any solution to the problem.
Ei minua haittaisi saada jotakin vaativampaa työtehtävää.
- I wouldn't mind some more challenging task.
Nyt, jos koskaan, on aika tarttua toimeen.
- Now, if ever, is the time to take action.
Elokuva oli parempi kuin odotinkaan.
- The movie was better than I expected.