Clash of Kings by George R. R. Martin
“And Arya, well . . . Ned’s visitors would oft mistake her for a stableboy if they rode into the yard unannounced. Arya was a trial, it must be said. Half a boy and half a wolf pup. Forbid her anything and it became her heart’s desire.
Catelyn talks about her daughter Arya, who she thinks is long dead. She remembers how she used to behave like a boy, not a proper lady. Catelyn says that Arya desired to accomplish anything that was forbidden by her mother Catelyn.
I understand the sentence in context, but I'm confused about this structure the author used. Is this some kind of clause here? -> Forbid ... and it became ... . What is the tense of forbid here and why is it used with past simple [became]?
I never encountered such a device. Can anyone help me deal with grammatically? Would appreciate any extra resources to read up on it.
Thanks in advance