When we ask for something, would and could are more polite than can, so when we ask someone to do something, we use would you or could you.

     Would you help us with a few things?
     Could you drive us to the park?
    

The following words often confused will assist not only knowledge and usage of words but also their correct spelling.

Some sets are synonyms (similar meaning but different usage); some are homonyms (same sound but different meaning); and others are similar enough in sound to be confused.  Words of opposite meaning, e.g. invaluable, valueless, are called antonyms.