Spell

'To spell' is an irregular verb. Its forms are given here:

Base form	past tense	-ed participle	Remarks
spell	spelt or spelled	spelt or spelled	the variation between 'spelled' and 'spelt' is a regular variant spelt is less common in American English. See note [b] in category:Quirk class 1 A There is also the derivative misspell

This is one of the "the 250 or so irregular verbs" listed in Quirk 1985. The list "contains most of the irregular verbs in present-day English ... but is not meant to be exhaustive, particularly with regard to derivative verbs." AWE has copied most of the entries in that list. The verb 'to spell' belongs to Quirk's Class 1 A

Note that there is also a kind of cereal grain called spelt.