(=give one's word) promettre
I'll write, I promise! J'écrirai, c'est promis!, J'écrirai, je le promets!
→ "You will go, won't you?" -- "I promise."
→ I'll be back at one o'clock, I promise.
to promise to do sth promettre de faire qch
→ I promised to take my children to the fun-fair on Saturday.
→ Jenny promised to be home early.
→ He promised to wait.
She promised to write. Elle a promis d'écrire.