Talking about perfective and imperfective verbs

Здравствуйте! I speak spanish, and I'm learning russian. We're studying perfective and imperfective verbs, like читать - прочитать. I get really confuse with some глаголы, like купить& покупать, i thougth they were imperfective and perfective respectively, but it turns out that is the other way around, is there a rule for this or it's just an exception?Then another глагол wich is confusing is говорить, and разговаривать, заговаривать, поговорить, I assume that all of this are perfective глаголы of говорить, but i don't know when to use them.There are other words like полюбить that i don't understand how to use, here i have an example from the excersize book: ХуанполюбилМарию, когдаонивместеучилисьвшколе, here полюбил is perfective, but the translation isn't clear for me, is it: "Juan loved María when they were studying together in the school"? or "Juan used to love María when ...." I don't know if you can tell the difference in english, but in spanish there's a difference. Hope you can help me, спасибобольшое.Thank you for your time!