Regardless of whether goals are achievable or well within management's perceptions of the
emplyee's ability,if employees see them as
unachievable they will reduce their effort.
Managers must be sure,therefor,that employees feel confident that their efforts can lead to performance goals.For managers,this means
that employees must have the capability of
doing the job and must regard the appraisal process as valid.这句话的主题句 应该是第二句 THERFOR 之后这句吧
怎么参考的说是第一句是主体句？

你说的对！这句应该是主题句：
Managers must be sure,therefor,that employees feel confident that their efforts can lead to performance goals.

第一句话不过是提出问题而已。THEREFORE这句才是本段要强调的主旨。最后一句是主题句的进一步解释。