In this sentence "the bus set off at 8 AM" that I read in a lesson, I don't understand why it is writtent "set off" and not "sets off". Thanks
I guess it's later in the day as someone says "the bus set off at 8 AM". Was the schedule reading 08:30, then the busman is guilty. Fortunately, it's not you who missed the bus; Nicolas.
"set" can be present or past. Only in present time the third person gets an -s. So when I read: the bus set off at 8 am, I hink the bus must have already left the station. Perhaps the following sentences in your text made that clear.