i don`t believe the reason ( )he has givem for his being late为什么不是用WHY,而是用THAT呢

i don`t believe the reason ( )he has givem for his being late
为什么不是用WHY,而是用THAT呢

从句中少了宾语所以要用that
也就是说:he has given the reason for his being late.==that(the reason)在从句中作宾语.
如果在从句中作原因状语,则用why
==This is the reason why he was late.

因为这不是定语从句，而是同位语