Undo

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

Base form	past tense	-ed participle	Remarks
undo	undid	undone	derived from Do (irregular verb)

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 undo' belongs to Quirk's Class 4 C c

Before its use in computers, the verb 'to undo' was probably most used (in the twentieth century anyway) as a mocking stereotype of Victorian and pre-Victorian morals: the passive participle in such sentences as "Alas! I am undone!", spoken by female characters in melodrama, indicated the loss of respectability consequent on the loss (or apparent loss) of virginity.