How do you remember all of the equations and what are the main or best ones to learn??
It depends some i just memorise, on some concepts i just derive out every time. Others i'll look up when needed. Mostly its a combination of memorisation & ability to derive out when required, if u use an equation enough times it just becomes embedded in my brain either way.
Every time you do a question that requires the equation, write the formula down. Every time. Every time. Every time. Eventually they'll be impossible to forget.