"learned" is much more common in the US than is "learnt". It may be different in other English-speaking countries, but that's the way it is in the States.
Actually, I think I can answer this myself.
The *prescriptive* answer is:
"learned" should used in phrases such as "a learned professor", in which case it is pronounced with two syllables.
"learnt" should be used in phrases like "I learnt a valuable lesson today".
The *descriptive* answer in British English is:
"learned" is used in phrases such as "a learned professor", in which case it is pronounced with two syllables.
Either "learnt" or "learned" are used interchangably in phrases like "I learnt a valuable lesson today".
The *descriptive* answer in American English is:
There is no such word as "learnt". Use "learned" always.
It depends on what impression you want to convey. There are plenty of descriptivists on this list who will tell that going with the flow is the thing to do. "Do what everyone does", and it will be "right" by definition. However - I would point out that if you are WRITING for a PROFESSIONAL JOURNAL (you didn't say professional, but even amateur journals would surely wish to seem professional) then you might want to come across as more, well, learned!
That said, the use of "learnt" in America seems to be almost zero even in professional journals, so this could be one of those areas where the "rules" are changing.
Or to put it another way:
Active verb in the past tense: I learned French.
Past participle: I have learnt French.
This word is going through change. The word "cookt" used to exist.
Active verb: I cooked a pie
Participle: I have cookt a pie.
However, "cookt" is now obselete. I fear "learned" is going the same as we all confuse "leanrt" and "learned".
Learned (meaning educated) is an adjective in its own right (like "intelligent").
That's all folks.
As an engineer, translator, and American, I use both learnt and learned as an adjective. For instance, a person is learned (two syllables), while scientific material is learnt. "The learnt material is very important for the exam on Monday." This, however, is not to be confused with the past participle, which is always learned (which is one syllable and has been used almost exclusively for most of the twentieth century on):
"I learned how to derive the Navier-Stokes equations, but the learnt method was dependent on several assumptions."
"I learned how to derive the Navier-Stokes equations, but method learned was dependent on several assumptions."
"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.
"Stephen Hawkins is a learned man." [present participle: "learned"; auxiliary verb: "is" (to be)]
"I have learnt a lot of thing 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 it's 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 it's 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.]
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