7. Mary's grandfather (hurt) his back while he (dig) in the vegetable garden yesterday.

{"english_answer"=>"Mary's grandfather hurt his back while he was digging in the vegetable garden yesterday.", "explanation"=>"This sentence describes an action that was happening in the past (digging) while another action (hurt) occurred. 'Hurt' is already past tense, so it stays as is, and the verb 'dig' needs to be in past continuous tense, so it becomes 'was digging'."}