Welcome to the forum, Gokul.
Please try to give your threads more helpful titles in future. 'Use of would and could' would have given us a much better idea of what the thread was about.
Also, remember to start your sentences with a capital letter and end with an appropriate punctuation mark. Always use a capital letter for the pronoun 'I'.
Your question is much too broad for us to answer in one thread. Have you a particular sentence in mind, where you don't know which word to use?