You're old fashioned
Over the 19th and 20th centuries,
spelt was much more commonly seen in British writing than
spelled. It wasn’t necessarily considered a mistake to use
spelled in Britain,
spelt just happened to be the preferred, more common spelling.
This preference seems to be waning, however, as
spelled has grown in popularity in Britain in recent years (see below).
Don't make this mistake ever again. Learn the proper spelling of spelled and spelt with example sentences and quizzes at Writing Explained.
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