Sentence:  Jubal 's conversations coming and going - Jubal 's words troubled him most ; he studied them , compared them with what he had been taught as a nestling , struggling to bridge between languages , the one he <WSD>thought</WSD> with and the one he was learning to think in . 
Meaning of the word marked with <WSD> tags:  use or exercise the mind or one's power of reason in order to make inferences, decisions, or arrive at a solution or judgments