present perfect is formed with -- have or has + past participle The past participle of learn is learnt or learned So present perfect is have learnt or has learnt eg We have learnt to be good. She has learnt to play the guitar
Present perfect is formed with - have/has + past participle The past participle of learn is learnt so present perfect is have learnt or has l;earnt. We have learnt to bake a cake. She has learnt nothing.
If you want to learn Ajax simply buy a book about it, spend some time on it (months) and hopfully you learn it. It's just like learning a new language. The most important thing to remember is that while learning anything the PERFECT practise makes PERFECT.