But at the movement’s root is a dramatic shift in identity since Britain pull
ed out in 1997 that has left a majority of Leung’s generation rejecting ties
to mainland China and asserting what they see as their own distinct and entire
ly separate identity.
很長有點高拐的句子但是這裡since做為連接詞表達“自從”意思時，開頭的子句不需要
使用完成式嗎？

誰說since後面必須接完成式的？沒這回事。