Could you please explain why “had attended” doesn’t work in this sentence?
If your late father {had been attending / *had attended} your wedding tomorrow, he would have been very proud.

Hi. Could you please explain why “had attended” doesn’t work in this sentence?
Thank you.

Question

Could you please explain why “had attended” doesn’t work in this sentence?
If your late father {had been attending / *had attended} your wedding tomorrow, he would have been very proud.

Hi. Could you please explain why “had attended” doesn’t work in this sentence?
Thank you.

Answer

Basic information about conditionals here:
https://www.ef.com/ca/english-resources/english-grammar/conditional/

Type 3 - hypothetical past
If your late father HAD ATTENDED your wedding YESTERDAY, he WOULD HAVE BEEN very proud. 
If your late father HAD BEEN ALIVE and HAD ATTENDED your wedding YESTERDAY, he WOULD HAVE BEEN very proud.

Type 2 - hypothetical present/future
If your late father ATTENDED your wedding TOMORROW, he WOULD BE very proud. 
If your late father WERE ALIVE and ATTENDED your wedding TOMORROW, he WOULD BE very proud.

Answer

'Had attended' (past perfect tense) is a completed action. The future hasn't happened yet.