Could is to would as can is to will. That is, could is the past tense of can, would is the past tense of will.
I think these words are possibly confusing due to their prevalence in 'perfect' tenses. Polite speech is often in these tenses - 'would you get this' rather than 'Will you get this', for example. I don't understand how that makes it more polite; to me, more polite would be, "Please get this". I especially am bothered with 'could you get this' or 'can you get this' in most instances in which people have used those to make requests of me. 'Could you answer the door?' for example. As a matter of fact, I've answered the door many times, I'm not crippled, I'm not tied up, and I'm dressed, so why are you doubting my ability to get the door? But that said, a lot of that is just me. I'm a bit autistic, so there's a lot of things about how words are used that don't entirely make sense to me.
I probably should have also asked what the past tense of can is if not could. I suppose it's not too late, so... What is the past tense of can if it isn't could?