有野問..thx.

1/ 應該係 She is one of the girls who... 而不是 She is the one of the girl...

2/ who=關係代名詞, 可用that取代, 用黎更詳細地形容前面girls, 讓讀者/聽眾知道說話既人唔係隨隨便便講緊某班女仔, 而係個班被人捉到偷野既女仔 (因為係一班所以加-s)
用既時候記住尾隨你要補充資料個隻字/詞就得啦~
The boy who wears a red shirt is my brother. (穿紅衫那個男孩)
Ms Chan, who is our English teacher, is retiring this year. (教我地英文個個Ms Chan)
My brother was one of the students that represented our school in the singing contest.

She is the one of the girl who were caught shoplifting last week.
我覺得呢句句子有問題，正確是

She is the one of the ( girls ) who was caught shoplifting last week.

one of the girls 一班女孩子中的一個

“Who” is a relative pronoun( 關係代名詞)，作用是更詳細地形容主名詞girls

e.g. The man who wears a T-shirt is my father.
這裡 “who wears a T-shirt ”更詳細形容是那一個人

因為係有一班女仔偷野......係呢度畈were係話緊果班女仔, 唔係淨係指某一個(she), 所以要用were
who=(用作限定關係代名詞)...的人
如:The girl who spoke is my best friend.
那講話的女孩是我最好的朋友。