Could

                                            Could
             
             It denotes the past ability.
    
                             could + baseform

        Examples:

                             I could speak with C.M.
                             I could give a speech.
                            They could win the match.
                            We could gather the people to MLA meeting.
                            They could participate the marriage function.
                            He  could attend the interview yesterday at right time.
                            When I was young I could swim the sea.
      
         Negative forms:

                           I could not achieve my goal.
                           They could not impress the people.
                           You could not understand those lessons.
                           Sachin could not play the last match.
                           We could not estimate well the movie.
                           Nikhil could not write the full lessons.

         Negative interrogation forms:

                            Could not you write the examination well ?
                            Could not you help the others ?
                            Could not I participate the last batch picnic ?
                            Could not they paint picture neatly ?
                            Could not she take classes yesterday?
                            Could not he eat the full meals ?

      Note:   Also use for requests people (more formal way)

               Examples: 

                              Could you please give me your pen?
                              Could you please move a while ?
                              Could you please understand my problem?
                              Could I speak to principal medam ?
                              Could I make reservation for Godavari Express?