I think it's "Winter is followed by summer" but our teacher taught us "Winter is followed after summer". I don't understand why would "after" be used after "followed", it just doesn't make sense to me. Any help would be appreciated.
Your teacher is incorrect.
Active: Summer follows winter.
Passive: Winter is followed by summer.
(Before proceeding, note that summer follows winter and that winter follows summer — it’s a hamster wheel kind of thing. Also note that, while summer does follow winter, it must first follow spring.)
The most interesting aspect of your question is its use of the verb follow, in this sense:
III. To come after in sequence or time and related senses.
16. a. transitive. To happen or occur after (something) in time; to come after (something) as an event; to succeed.
Source: Oxford English Dictionary (login required)
In this use, the by-agent is not optional; you must include it:
Winter is followed by summer.
*Winter is followed. (incorrect)
1What happened to spring? Feb 19, 2022 at 10:33
@MichaelHarvey: In early Old English, there were only two seasons, winter and summer. The reason we have four now is that the Catholic church arrived, and it used four seasons, like the Romans did. His teacher is well over 1000 years out of date. Feb 19, 2022 at 13:08
(simple present tense) Active voice: Summer follows winter.
(simple present tense) Passive voice: Winter is followed by summer.
(Active: A singular verb follows a singular subject.
Passive: A singular subject is followed by a singular verb.
Active: P follows O.
Passive: O is followed by P.)