Swan的"Practical English Usage"有提到这。他说这用法"...This is unusual"(一般过去完成式是为了点出动作发生比另个动词(此处是主要子句的动词expressed)更早。但是此处是...“更晚”,所以"unusal")此句的"had begun"是为了强调这动作“已经完成”。不想刻意强调(只是寻常叙述)就写 began"...都中文逻辑在思考"-> 语感不足之前这没办法。现在碰到这新的你之前没碰过的东西就学起来比如这个:
http://tinyurl.com/glwb22nEven before ... began .., .... began to ....
http://tinyurl.com/h9xgyck 找 Even before thenegotiations began, ... drew ....