Ok, lets see if I remember any french....
Would is the Simple Past of will
It is used to build the conditionals , eg. I would go Madrid, if my parents let me.
could=past Simple of can =pouvoir= je pouvrais
eg. I could go to the movies, if I had the money.
So with could+would you build the conditionals.
can suggests ability eg. I can drive my dad's car. I can speak German.
or asks for permission Can we play in the garden?
May the most polite way to ask for permission eg. May I (do I have your permission ?) smoke?
Sometimes it suggests that something is probable. eg I may go out tonight. (I don't know for sure, it is possible)
Ought +to+verb =must eg. You ought to stay in bed since you are sick (but you did't)
Should , the same example applies here too ,You should stay in bed since you are sick. (it is my advice)
Hope I have been of some help.
Student or Learner