You can say "bears hibernate in winter" or "bears hibernate in the winter", and it's true that you can't say "the winters" here.
But I think it's different when you're talking about specific winters. I think it would be quite natural to say, "The winters in Tibet were harsh" if you meant, for instance, "The winters that we spent in Tibet were harsh."
For the same reason, I think "We swim in the summers" makes sense in English if the context is something like, "I've lived in Norway for four years. We swim in the summers (that we have spent/will spend in Norway) and we ski in the winters (that we have spent/will spend in Norway)."
Another example: sometimes it makes sense to say "the Mondays" in English. "My coach has created a training program for me with something to do on every day of the week. I run on the Mondays (that are in the program) and I swim on the Tuesdays (that are in the program)."
What I don't know, though, is whether "the summers" in Norwegian has a similar meaning (ie, you can only use the bestemt form when talking about specific summers), or whether "somrene" just means "summertime" generally.
Ok, I noticed someone asked the same question three years ago without getting an answer, so here's a quick summary from "Norsk grammatikk" by Kirsti MacDonald. Apologies in advance for typos and dodgy translations, etc below (which there will be plenty of), and for not using a Norwegian keyboard (hopefully this won't be too confusing...).
Firstly, there are prepositions to do with time and prepositions to do with place, and also prepositions to do with more abstract circumstances. I'll just summarise the section to do with TIME (though the difference between "i" and "pa" where place is concerned is also confusing).
We use "i" in many time expressions, amongst them:
-- Period (present, past and future): "Vi har vaert her i ei uke" ("We've been here for a week"). "De skal bo der i mange ar" ("They'll live there for many years").
-- With months. "Vi skal flytte i august." (We'll move in August.)
-- With years in number form. "I 2009 flyttet vi til Bergen." ("In 2009 we moved to Bergen.")
-- With centuries (but see also "pa" below). "Norge fikk sin grunnlov i det 19. arhundre." ("Norway got its constitution in the 19th century.")
-- With holiday periods. "I paska gar de pa ski." ("In Easter they're going skiing.")
-- Seasons and times of day. "I host skal jeg trene mye." ("In autumn, I'll train a lot.") "Hvor skal du i kveld?" ("Where will you go in the evening").
We use "om" when we tell how long it is till something happens in the future, or to express that an action repeatedly happens.
-- Future. "De kommer om tre dager." ("They're coming in three days.")
-- Repetition. "Jeg liker meg ikke om vinteren." ("I don't like it in winter.") "Jeg liker a vaere hjemme om kvelden." ("I like to be at home in the evening.")
We can use "pa" before a word for point of time.
-- With the word "hundretallet". "Norge fikk sin grunnlov pa attenhundretallet." ("Norway got its constitution in the 1800s.")
-- Days. "De kommer pa mandag". ("They're coming on Monday.")
-- Completed action. "Hun malte huset pa tre dager." ("She painted the house in three days.") Or talking about how long it's been since something happened. "Jeg har ikke sett dem pa to uker." ("I haven't seen them for two weeks.")
-- Repetition. In spoken language, you can use "pa" instead of "om" to express repetition. "Jeg jobber pa formiddagen." ("I work in the afternoon.")
We use this when we say how long it's been since something's happened in the past.
-- "De kom hit for to ar siden." ("They came here two years ago.")
We can use til about the future.
-- Seasons. "Jeg kommer hjem til sommeren." (I'm coming home for the summer.")
-- Holidays. "Jeg kommer hjem til jul" ("I'm coming home for Christmas.")
We use "siden" with nouns that express point of time. The verb is in the present perfect.
-- "Jeg har vaert her siden 2010." ("I've been here since 2010.") "Hun har vaert syk siden sondag." ("She has been sick since Sunday.")
But if we want to say how long a period is, we have to use "i".
This is used with nouns that tell about a period of activity, but which don't talk about time (like "summer", "weekend", "holiday").
-- "De var her under hele krigen." ("They were here for the whole war.") "De kjedet seg under konserten." ("They were bored during the concert.")
I LOPET AV
-- "De malte huset i lopet av ferien." ("They painted the house in the course of a week.") "De laerte mye i lopet av kurset." (They learnt a lot during the course.")
FØR, INNEN, ETTER
-- "De kommer før sommeren." ("They're coming before summer.") "Du ma vaere ferdig innen tre maneder." ("You must be finished within three months.) "Vi reiser etter jul." ("We're travelling after Christmas.")