Use of Models

I came to know that in english models are used depending upon the time & mood of the situation.As we use different helping words in our mother tongue.However i did not come to result where to use which model.It is very confusing because in same sentence we can use the "will" or "Would' .It does not make the difference in form of sense either we use "will" or "would".There should be some definite rules or regulation in english language so that english language could be understand.If anybody have any streamlined rules to use these models please share with me.This happen in other models also like can could ,may might ,shall will etc....