Open the brackets and write the sentence using Present Simple or Past Simple. Dad ... (not/buy) a new car last week.

The sentence requires the Past Simple tense because of the time expression "last week". The verb "buy" is irregular, and its past tense is "bought". Since the sentence is negative, we use "did not" or "didn't" followed by the base form of the verb "buy". Therefore, the correct form is "did not buy".

Dad did not buy a new car last week.