can you teach me how to use properly the could & would words?
thanks in advance.
Degree of Politeness Auxiliary
somewhat polite: "Could you tell me ...?"
quite polite: "Would you tell me ...?"
The modal auxiliary could is used
 to express an ability in the past:
ex: I could always beat you at tennis when we were kids.
 to express past or future permission:
ex: Could I bury my cat in your back yard?
 to express present possibility:
ex: We could always spend the afternoon just sitting around talking.
 to express possibility or ability in contingent circumstances:
ex: If he studied harder, he could pass this course.
Note that, in expressing ability, can and could frequently also imply willingness: Can/Could you help me with my homework?
The modal auxiliary would is used
 to express willingness:
ex: Would you please take off your hat?
 to express insistence (rather rare, and with a strong stress on the word "would"):
ex: Now you've ruined everything. You would act that way.
 to express characteristic activity:
ex: After work, he would walk to his home in West Hartford. <customarily>
ex: She would cause the whole family to be late, every time. <causal>
 to express a hypothetical meaning:
ex: My cocker spaniel would weigh a ton if I let her eat what she wants.
 to express a sense of probability:
ex: I hear a whistle. That would be the five o'clock train.