The use of 'learned' and 'learnt'

Today, when I was revising my statement of purpose (SOP), I felt confused how to use the word 'learnt' correctly, and I think the information in this post is more convincing.

http://answers.yahoo.com/question/index?qid=20070317112122AATO9XB

In short, 'learnt' is the past tense of the verb 'learn' in British/European English, whereas 'learned' is the past participle of the verb 'learn' and (sometimes) can regarded as the past tense verb in American English.

BTW, you cannot use 'learnt' as an adjective neither.  Use 'learned'.

For example:
I want to learn French. (correct)
I learnt French last summer. (correct, more British)
I learned French last summer. (ok in American English)
I have learned French since last summer. (correct)
I have learnt French since last summer. (INCORRECT)

To sum up:
learned: past tense verb (American), past participle, adjective.
learnt: past tense verb (British/European).

I would like to use them in this way:
simple present verb - learn
simple past verb - learnt
past particle - learned
adjective - learned

Feel free to correct me if there is any misunderstanding, thanks!