I haven’t visited my grandparents since the COVID-19 pademic broke out again.  I last______________________________________________________________

I haven’t visited my grandparents since the COVID-19 pademic broke out again.

→ I last visted my grandparents was in the COVID-19 pademic broke out again.

⇒ S + have/ has + not + for + khoảng thời gian 

                                     since + mốc thời gian

= S + last + Ved + (was) + time+ ago: Lần cuối cùng làm gì

→ Vì là chỉ mốc thời gian ( về đại dịch COVID-19 ) nên ta dùng ‘was’