Re-posting with corrections (and requesting the moderators to please allow this):
English (as in Queen's English):
"learned": a present participle that performs the role of an adjective by qualifying a following noun.
"learnt": a past participle that performs the role of a adjective by qualifying a noun.
These words will be participles only if used along with a helping verb, also called an auxiliary verb like "to be" or "to have." If used without an auxiliary verb, there is a possibility that the word "learnt" is actually a verb and not a participle. This depends entirely upon the sentence structure.
Both these words are derived from the infinitive of the verb "to learn." While "learned" refers to a current state of acquired knowledge of the accusative noun, in this case the the noun following the word "learned;" the word "learnt" refers to a past incident that caused the accusative noun to become aware of something or gain some knowledge.
Examples:
"Stephen Hawkins is a learned man." [present participle: "learned;" auxiliary verb: "is" (to be)]
"I have learnt a lot of things by attending this class." [past participle: "learnt;" auxiliary verb: "have" (to have)]
"I learnt about it last night." [verb: "learnt;" auxiliary verb: none, not required, because "learnt" is a verb in its own right]
If the sentence "I learnt about it last night," sounds confusing as to why "learnt" is a verb, try rephrasing it as "I did learn about it last night." Although there is a subtle difference between the two sentences, they convey the same meaning.
N.B.: North American English is rather "liberal" in its usage of vocabulary and grammar, many of which may be considered wrong according to Queen's English.
Hope this helps.
[Pardon me if there is any grammatical mistake.]
-Regards-