1a) By that time, he was exhausted.<br/><br/>or<br/><br/>1b) By that time, he'd been exhausted.<br/><br/>2a) He finished his work by 7'o clock.<br/><br/>or<br/><br/>2b) He had finished his work by 7'o clock.<br/><br/>Why is 1a correct and not 1b, when in the 2nd example 2b is the correct one?