to swear, vow (promise solemnly)
vannoa vala ― to take an oath
vannoa kostoa ― to vow (to get) revenge
vannoa luopuvansa (+ elative) ― to swear off
vannoa [+ genitive +] nimeen ― to swear by
vannoa uskollisuutta (+ allative) ― to pledge allegiance to
vannoa väärin / vannoa väärä vala ― to perjure oneself, commit perjury
Olisin voinut vannoa, että...
I could have sworn that...