pin someone down
phrasal verb with pin verb uk/pɪn/ us/pɪn/ -nn-
(SURROUND)
使（某人）动弹不得;困住（某人）
to stop someone from escaping by surrounding and shooting at them if they try to escape
[ often passive ] Government forces were pinned down by resistance fighters 30 miles north of the capital. 政府军被抵抗组织战士钳制在首都以北30英里处。
(GET DETAILS)
（使）（某人）明确说明;（使）（某人）作出决定
to make someone provide details about something or make a decision about something
I've tried asking Stephanie, but she's proving difficult to pin down to a particular date. 我已经问过斯蒂芬妮了，可她很难确定具体日期。