If you're following UK writing conventions, use learnt.
If you're describing someone as educated, you must use learned.
Learned and learntThe verb to learn means to acquire knowledge of, or skill in, something through study or experience.
To learn is one of those verbs with both an irregular form and a regular form. (See the table below for some others.)
The past tense and the past participle can be written as either learned or learnt. However, they are not interchangeable, especially in the US.
Americans Demand LearnedIn America, learned dominates. The use of learnt as the past tense or past participle of to learn is considered a spelling mistake by many. It will certainly annoy a fair proportion of your readers.
Brits Prefer LearntOutside America, learnt is more common, but learned is generally accepted. (This is almost certainly a result of American influence spreading.)
Verbs with Irregular and Regular FormsThe following verbs (like to learn) can be regular or irregular:
|Verb||Past Simple||Past Participle|
|burn||burned OR burnt||burned OR burnt|
|dream||dreamed OR dreamt||dreamed OR dreamt|
|learn||learned OR learnt||learned OR learnt|
|smell||smelled OR smelt||smelled OR smelt|
|spell||spelled OR spelt||spelled OR spelt|
As with learnt, the second form (e.g. spelt, dreamt) is more common in British English.
The word learned is used as an adjective meaning knowledgeable, wise or well-educated. It has two syllables and is stressed on the first one (i.e., LEARN + ed).