Think through(仔细考虑)>think up (想起)>think back(苦苦思索，回忆)
1. to believe something or have an opinion or idea 相信;觉得;思考
[ + (that) ] I think (that) I've met you before.
I don't think I will get the job.
"Do you think (that) you could get me some stamps while you're in town?"
2. to use the brain to decide to do something 思考，考虑(动脑筋决定做某事)
[ + of+ -ing verb ] I'm thinking of taking up running.
[ + (that) ] I think (that) I'll go swimming after lunch.
I'm thinking about buying a new car.
3. to use the brain to plan something, solve a problem, understand a situation, etc. 考虑，想，思索
What are you thinking?
He just does these things without thinking and he gets himself into trouble.