"I believe in summer."
Translation:Jag tror på sommaren.
This innocent-looking and slightly baffling sentence is actually a cultural reference embedded by the creators of this course to lure us innocent students directly to the hard core of swedishness: https://www.youtube.com/watch?v=Y_7UYRQTHjc
Both this song and the event "Allsång på Skansen" are as Swedish as anything can be, leaving ABBA, Ikea, kanelbullar, and surströmming far behind.
Though not obvious from the example, this is an expression, taken from a 1960s song. That said, this is arguably also one of the cases where Swedish requires a definite article whereas English does not. It's certainly up for debate. In some contexts I'd use the indefinite form, but not very often. So all in all - a very valid question!... without a 100 % correct answer. :)
Believing in several seasons is not mutually exclusive. I believe many appreciate there are four distinct seasons in Sweden, and their different aspects.
Swedish winters are such a palpable, inescapable, and sometimes harsh reality, that there are no doubts winter is coming every, single year. In a season, where the sun doesn't rise at all for almost a month in the northernmost part of Sweden, and even Skåne only has some 7 hours of daylight a day, it is understandable there are poetic sayings and songs about believing in summer. Not only that it will actually come, but also the promises it brings, a more relaxed way of life, a time when anything is possible. Swedes really flourish in summer, even though many of course also appreciate a snowy winter. Winters are more introverted, people are cocooning in their homes, lighting candles, cozying up with a cup of hot beverage in the sofa, while summers are more extroverted and socially active. This is of course an oversimplification, but I think it holds some truth to how life is at these northern latitudes. :)