The best time to start thinking about possible careers is while you are still at school, before you make any choices about your future education.
参考译文:开始考虑你可能的职业生涯的最佳时机是在你还在学校的时候,在你对你未来的教育做出任何选择之前。
备注(1):以上句子主干是:The best time is while you are still at school.
备注(2):以上是while引导表语从句。