There are a few sets of words that have two forms of the past tense and learnt and learned are one set. Two more examples are spelled/spelt and burned/burnt.
From much research today, it appears that learned and learnt are two forms of the past tense of the verb to learn. I have found a few forums and other websites that try to differentiate between the two, some saying that learned is the past tense of the verb to learn and that learnt is an adjective, but nothing appears consistent.
According to my big Collins English Dictionary & Thesaurus learnt is the past tense and past participle of learn, but then under the entry learn, it shows the past tense as either learned or learnt.
Collins English Dictionary & Thesaurus also has an entry for the adjective learned [pronounced learnid] e.g. my learned friend.
My thoughts: I tend to use learnt as the past tense in what I write, for example, at work I have recently completed a ‘lessons learnt log’ for my end of project report. I will continue with my research, but generally it appears acceptable to use either learnt or learned as the past tense – just remember to be consistent in what you use.
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Collins English Dictionary & Thesaurus