请问,
I would now like to introduce my friends to you.
This is John, Paul, Tim and Bill.
At this last name Susan, who had until then had bowed courteously
^^^^^^^^^^^^^^^^^^^^^^^^^
but coldly and impassively, involuntarily took a step forward.
若文法没错，had until then had bowed看不懂为何如此表示
^^^ 请教该如何解析
谢谢