I have been reading a few old novels (18th 19th Century) and have noticed something which I frequently have a problem with. I tend to have difficulty working out if certain words should have -ed or -t...ie spelled or spelt...it may have arisen from reading such novels when younger, but it does occur for more words (unspoilt, spoilt, dwelt etc) and I was wondering if you knew if there was a rule for it or if it is just another word/spelling shift?

----The next sentence is true. The previous sentence is false----