come about: 发生，happen, occur 的同义词
How does the bruise on your leg come about?
你腿伤是什么回事？

pull the trigger: 使发动起来
I realized then: He had never expected me to pull that trigger. In his wise, deep way he let me face my decision on my own.
这时我才明白，他从未想过我会扣扳机。他用明智、深刻的方式让我自己做出决定。
Pull the trigger,ie fire a gun
扣扳机(开枪).

tell the truth: 说实话
If I don 't tell the truth , can I avoid flat-out lies?
如果我不说实话，我能避免纯粹的说谎吗？
If I were you I would tell the truth, or you'll be dismissed.
如果我是你，我就讲真话，不然你将被解雇。