Because stopping something else is not a reflexive verb, but a transitive one, as it takes a direct object. In this sentence, time is stopping itself (reflexive). In the sentence with captain stopping the ship, the captain is the subject and the ship is the direct object (transitive). In that case, you use "avere" and the verb is "fermare" not "fermarsi".
Here you have some info about grammar. For info related to this topic in particular read the section named "Compound tense auxiliary verbs". http://www.thefullwiki.org/Italian_grammar.
Also search for this PDF document in google. It has a chapter dedicated to verbs which explains when to use "essere" or "avere" as auxiliary verbs.
GENESINI, Grammatica italiana in sintesi, Padova 2010. 1. Pietro Genesini, Grammatica italiana in rapidi schemi, Padova 2017
My question wasn't about the litteral meaning: in Italy we say "il tempo scorre" (the time goes by) and if it can "move" it can "stop" not litterally, but as a methaphore or as a personal sensation ("come se il tempo si fosse fermato" = as if the time stopped) or in any fantastic novel where time could stop. It (grammatically) can stop as far as it normally pass, run, go by, etc.
In Italian we can't say the same for the weather, we don't use movement verbs to describe the usual actions of the weather (we don't say "il tempo meteorologico è passato, corre, rallenta" etc.), so we can't use the verb "stop" for indicate its unusual or impossible action. (just some vernacular uses "fermarsi" with this meaning, but it isn't proper Italian).
Actually, we use the term "tempo" with the meaning of "weather" only as the subject of "essere" and some particular verbs :
il tempo è bello/brutto, il tempo migliora/peggiora, il tempo cambia, il tempo si calma, il tempo si mantiene, si guasta, resiste, regge, se il tempo permette (when some human action requires a particular weather), il tempo regge, il tempo tende al brutto/bello.
In all the other sentences where "tempo" is the subject, it means "time".
I disagree. Standing still and stopping are, if only by nuance, different things. The latter implies a sort of movement that took place before, the former does not. I think it feels like the same here because we are used to time "moving" and thus take the stopping for granted when it's standing still.
I have to agree with Gordon and Ellen on this. Although "Time stopped" is a more literal translation, the phrase "Time stood still" is the common English idiom describing the sensation that time has paused its relentless advance. If Ellen is correct that this phrase is common enough in Italian to be the title of a movie, then the best idiomatic translation into English is "Time stood still."
"si" can also be a form of passive, not necessarily a reflexive pronoun. this is called an "impersonal pronoun". more info: http://onlineitalianclub.com/free-italian-exercises-and-resources/italian-grammar/italian-grammar-si-imperonale/
no, the impersonal form is not passive!
"si fa qualcosa" = "we do something" or "someone do something" or "people do something". It has a passive form but an active meaning, so you have to translate it with the active meaning. AND the impersonal form doesn't have the subject: grammatically the subject coincide with the object ("si fa qualcosa" is litterally like "Something does itself"), the meaning is, indeed impersonal, without a specific subject. So if "si" is in a phrase with the subject it can't be an impersonal form.
I'm not expert in English.. but if someone says to me "time has been stopped", I'd find it hilarious: by whom? who has stopped the time? anyway in Italian it should be: "il tempo è stato fermato"
I appreciate all of the time people have invested in explaining this sentence and why proposed alternatives are not acceptable, but the fact remains that this is a poor choice for a pedagogical example. I say this because time does not, literally or in fact, stop. Therefore it is unsurprising that as students trying to learn, we would all be looking for other plausible meanings, such as bad weather ending, or the clock time stopping in a sporting match. And if it were intended to be instructive of an idiom, the only sensible equivalent in English must be "time stood still," which is apparently not accepted by the app. (By the way, is this idiomatic? The discussion wasn't clearly definitive about that.) This sentence should be removed and replaced with something like "il motore si è fermato" or "l'orologio si è fermato."