I faced lots of problem in this case so please help me that how can I understand that where could will be used and where was or were able to be used?????

These words are used differently in sentences, so it's important to know when to use were vs. was. Was is used in the first and third person singular past. It is used for statements of fact.Were is used in the second person singular and plural and first and third person plural.

Could" is used to express possibility or past ability as well as to make suggestions and requests. "Could" is also commonly used in conditional sentences as the conditional form of "can." Examples: Extreme rain couldcause the river to flood the city.