What are 'perfective verbs'? I googled for a while and can't really find the difference between należć and znależć (or robić and zrobić) ?
"naleźć" doesn't exist. The imperfective is "znajdować", but it's not used often - when you find something, you do it succesfully, so unless you find something very often (znajdujesz coś bardzo często), that won't be used often.
Polish language calles the perfective/imperfective differences "dokonany/niedokonany", which could be translated as "accomplished/not accomplished". So if it's important that you did something succesfully and til the end, you use perfective. If it's not important or you're talking about something that happens generally/often/rarely etc., that's imperfective.
"I was making soup" = Robiłem zupę. (Either I didn't finish or it's unknown/unimportant) "I made soup" = Zrobiłem zupę. (I finished doing it, I accomplished it)
"Naleźć" absolutely exists, e.g. "Znów tu nalazło to tałatajstwo", "Ale się do was nalazłem. Co za odludzie!" :) My English does not allow me to explain its meaning :)
that might be because verb "naleźć" if even exists has no connection in meaning to "znaleźć".
imperfective form of znaleźć is "znajdować".
Robiłem zadanie= I was doing my homework, it does not mean I finished doing it.
Zrobiłem zadanie= I have done my homework/ I did my homework- It is now completed.
With most verbs difference is between doing something, and something being finished, that of course is complicated with "find", because finding is usually one-time finished action.