I was dismayed to read a (closed) thread in which a number of members agreed that the words "learnt" and "learned" were interchangeable, having identical meaning. This is simply not so.
Well, actually, it is so. Some native speakers, especially of BrE, use 'learnt'. Other native speakers, especially those of AmE use 'learned'.
To avoid any possibility of confusion, we are discussing the verb "to learn".
Or, as some of us would put it, 'we are discussing the verb LEARN'. There is no need for a 'to'.
"Learnt" is the past participle, except for those people for whom 'learned' is the past participle.
It is used in the Present Perfect tense e.g. 'John has learnt something new today" or the Pluperfect tense (more commonly known as the 'past perfect'), e.g. 'John had learnt something new today'. Without further context, that is a strange use of the past perfect.
"Learned" is in the Imperfect tense, e.g. 'John learned something new today'.That tense is generaly known as the past simple/simple past. For most grammarians, English does not have an imperfect tense. The form of the past simple is 'learnt' or 'learned'. Almost all native speakers use the same form for the past simple as they use for the past participle.
Its use as the past participle is incorrect.
No it isn't.
ps. Welcome to the forum, Ian.