Learned VS Learnt

What is the difference between "learned" and "learnt", and when should one be used instead of the other?

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.

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. next time..