Hear ye, Oh hear ye!
Even though I am aware that language is a living thing with regional and temporal variance, I henceforth declare:
When rowing one's boat, one does so merrily - because life is but a dream.
When saying unto thee, one does so verily - because thou shalt tell the truth.
When one's mind is occupied with something, it could be preying on one's mind or, if a lighter thought, playing on one's mind, but it will never pray on one's mind.
Once said disturbance of thought has been dealt with, one shall have peace of mind. One may need to give a piece of one's mind to someone else, particularly if they commit a faux pas.
Chris may own whatsoever she likes, and I am willing to be flexible about her use of apostrophes, particularly if it affects the flow of the sentences. However, the issue of whether she owns many knives or knifes is currently in flux and is one of the many cross-Atlantic modern/traditional quagmires of language development.
I do hope that I have spelt all of this correctly.