Read each sentence to find out whether there is any error in any part When I finished writing the letter,(a) I could not help admiring myself to have(b) achieved the impossible(c). No error.(d)

The modal ‘Could’ is used to express possibility. It is simply used to state one or more things that are possible or were possible in the past. In this statement, ‘to have achieved’ needs to be replaced with ‘for achieving’ to make the sentence correct. Thus, the correct sentence would be, ‘When I finished writing the letter, I could not help admiring myself for achieving the impossible’.