Q:

Order to learn a subject (self-learning)?

There are a lot of subjects that to understand them you need to have other basic knowledge. So, let's say I want to study for some specific reason, the subject of abstract algebra.
Could this question be accepted?

What should I learn before abstract algebra to understand it?

A:

This looks broad and under-specified. What does it mean to "understand" "abstract algebra"? 
We can enumerate various mathematical fields that could be classified under "abstract algebra". Is your plan to write a Ph.D. thesis in each of them? Probably not. 
Perhaps "abstract algebra" means "a course by this name taught at my institution", in which case "to understand" probably means "to do well in said course". Then the question falls under "academic advising" category, because it is specific to that course. 
A course may be named "abstract algebra" but the course content and expectations vary dramatically from one educational system to another; a lot of undergraduates studying abroad find this out the hard way. 
So, the answers would be guesses based on each answerer extrapolating from their own experience to the situation of someone on the Internet. And I'd be voting to close. 

