A correspondent writes to say : 'I discovered that the verb to spoil has preterite and past participle spoiled but that it also has spoilt (not sure whether one is British and the other American). What I want to know is whether boil works the same way.'
I can find no trace of the boilt spelling in standard English. There are no instances in the OED, although there was a great deal of spelling variation in the early centuries of its use. But regional dictionaries show examples, especially in Scotland, Ulster, the Isle of Man, and parts of the USA (especially those influenced by Scots-Irish). A Scots poetic example from 1790: 'Twa pints o' weel-boilt solid sowins' [an oat-meal beverage].
With verbs which have two -ed forms, such as spoiled and spoilt, the situation is interesting and not entirely understood. The -t ending is rare in American English, certainly. In British English, an aspectual distinction is usually involved. The -ed form is used when the duration of an action or the process of acting is being emphasized, and the -t form when something happens once, or takes up very little time, or the focus is on the result of a process rather than on the process itself.
To test this hypothesis, the best way is to compare pairs of examples. In The house burnt down, the implication is that the event took place quite quickly, whereas burned is more likely in The house burned for days. Similarly, I've dreamed all my life of living in Scotland is more likely than I dreamt all my life of living in Scotland. Dreamt tends to be used for single, short, and determinate instances of dreaming, where the dreamer is asleep (I dreamt last night I was in Italy); dreamed tends to be used for a more continuous and indefinable dreaming, where the dreamer is awake (I dreamed of meeting you all week). There is some overlap, though not in contexts where the 'awake' sense is clear, such as day-dreaming, which gives rise to I day-dreamed, not I day-dreamt. It is not a hard-and-fast rule, but it does help to explain the relative frequency of different items. Spilt is much more likely than spilled because the action of spilling is usually short. Learned is much more likely than learnt because the action of learning usually takes some time.