For those stuck on rank 7, use this site: http://words-with.com/k-and-w.html
What I did was got the prefix for- and used these words:forebackwardly, foreknew, foreknow, foreknowable, foreknowableness, foreknower, foreknowing, foreknowingly, foreknowledge, foreknowledges, foreknown, foreknows, forkwise, formwork, formworks, forswink, forswinked, forswinking, forswinks, forswonck, forswunk, forwake, forwaked, forwalk, forwelk, forwork.
Hope this helped.