'Could' means 'can', but also implies the situation is hypothetical.
Ex. "I could scale that cliff." = "I can scale that cliff (if I wanted)."
Unlike 'can', 'could' can be used in past tense.
If you should do something, then it is the right decision.
Ex. "You should study more often."
'Would' is used in hypothetical situations (in all tenses). It is sometimes considered the past tense of 'will' if that helps. It describes what you would (will) do in a certain circumstance.
Ex. "I would've been badly hurt if I hadn't reacted in time."
The standard form is "__would..., if...", but often the "if..." is just implied.
Ex. "Would you like some tea (if I gave you some)."
"I would not do that (if I were in that situation)."
'Might' and 'may' express that the possibility of something happening exists. They are similar to 'can' and are often interchangeable when describing non-human objects. 'Can' represents the ability to do something, while 'might/may' only describes the possibility.
'May not' and 'Might not (mightn't)' don't mean that something is impossible, but rather that it's possible that something won't happen.
Ex. "It might/may rain today."
"It might/may not be sunny today."
"It might/may be night" = "It might/may not be day."