Forbid

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

Base form	past tense	-ed participle	Remarks
forbid	forbad (IPA /fə 'bæd/) forbade IPA /fə 'beɪd/ OR /fə 'bæd/)	forbidden	See also bid

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 forbid' belongs to Quirk's Class 4 B c

Quirk allows two spellings in past tense: forbad and forbade. The first is pronounced like 'bad' (IPA /bæd/); forbade has the vowel of 'save' IPA /beɪd/. Both LPD and OED prefer 'bad'. So does AWE. The spelling forbade (with the '-e') seems generally preferred.