I think you should consult dictionaries first, but I am not a teacher.
Student or Learner
when or how should be use will, would, can and could?
Note my corrections above. You must start a sentence with a capital letter and you should mark out the words you are querying in some way. I have changed them to italics. You could put them inside quotation marks too.
Your question is far too broad to be answered on this forum. We do not replace the study of English grammar. We enhance it, answering specific questions you have once you have mastered everything you can using home study or by attending classes.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.