You've met it again and again: Helsinki → Helsingissä, ottaa → otan, pöytä → pöydällä. This is consonant gradation (astevaihtelu). Certain consonants in the middle of a word have a strong and a weak grade:

Strong	Weak	Example
kk	k	pankki → pankin (bank)
pp	p	kauppa → kaupassa (shop)
tt	t	tyttö → tytön (girl)
k	— (vanishes)	jalka → jalan (leg)
p	v	leipä → leivän (bread)
t	d	katu → kadulla (street)
nk	ng	Helsinki → Helsingissä
nt / lt / rt	nn / ll / rr	ranta → rannalla (shore)

The dictionary form is (usually) the strong one — so you always start from something you can look up.