I will give you an answer immediately I've finished reading your report. immediately副词,在本句话中作连词用,引起一个时间状语从句,按照一般正常情况,应该按照"主将从现"原则,也就是说I've finished 应该用现在时 I finish⋯。汉语意思:我一完成(现在时替代将来时)... ,就给你回话。而用现在完成时,则表示替代将来完成时。意思是强调在将来完成这个动作的那一瞬间(毫不耽误)给你回话。追答
当然,如果你也可以把从句改为... immediately I've finish reading your report.