i guess a definition of "learn" could be to have a solid understanding of the "body method" and principals of the form and to be able to apply those concepts in fighting. this might sound weird but i do not think you must master each and every move of a form to really understand its body method and to apply the concepts of the form in fighting, there are a few moves in my forms i just do not do well but my overall understanding of the concepts the form is teaching me is still there ...to add to that i think a "form" is only a tool to teach you body a way of moving ...
i am sure many people who have just 2 forms might not understand how to apply them ...
in the last 10 years of training i have learned about 15 martial art forms (some very long and complex and some short) and about 6 meditation/chi kung type of forms. i think i have some skill and understanding with 3 martial forms. but the others have had a positive influance on my over all skill and with continued practice i think i will gain skill with them too ... (i hope ... lol )