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A new textbook focused on recent advances in artificial intelligence
was assigned by out instructor
How do i know if Focused or Assigned is the past tense verb? Why can't i say A new textbook
was focused on recent advances in artificial intelligence assigned by out instructor?
I also initially thought the subject was textbook and verb was focused. I thought a textbook could do the action of focusing..
To answer your questions..
1) focus, assign are base form of verbs and when you add 'ed' to them ,they become past tense. Most of verbs follow the same rule.
2) in the initial sentence the statement could be ...
A new text book ( focused on....) was assigned by....
So focused on.. works as a we modifier modifying the textbook..
In the sentence you have made..
Assigned by... Will modify artificial intelligence and would mean artificial intelligence that was assigned by .. completely wrong
It would be good to go through some good grammar book for understanding all the concepts of tense etc..