"I somras" is used when talking about the past. I personally use "i somras" when talking about the last summer in the past, "förra sommaren" when it's summer and I'm talking about the last summer, "den här sommaren" when talking about this summer in the future or present tense, "nästa sommar" when talking about the next summer. "Jag hade mycket roligt i somras" sounds the best to me. The year doesn't really matter, but if it's before, in or after the summer. (and which summer you're talking about) "I sommar" is used in the future tense. "I usually go to the seaside in summers" would be something like "på sommaren brukar jag åka ut till kusten" or "jag brukar åka ut till kusten på sommaren", depending on the emphasis. I guess "på sommaren" is used when "i somras" and "i sommar" aren't.

Well, I can't really agree with Cyaoeu that the year doesn't matter. 'I somras' has other parallells, e.g. you can say 'i lördags tittade jag på tv', which means 'last Saturday I watched tv'. All these varieties always means the most recent day/season you're talking about. You wouldn't say 'i somras' about a summer two years ago (or ten years ago). Basically, you can use it up until the next summer starts, which kind of depends on your sense of when the seasons change, and it's always past tense. So in July you can't use 'i somras' to talk about what happened i June. I'm writing this in January 2011, so 'i somras' would have to refer to the summer of 2010, and no other summer. There's also the similar phrasing 'i morse', which means 'this morning', but always in the past tense. My fiancée (from Ireland) get's confused abouth that, and sometimes say for example 'jag är så trött i morse' (she's trying to say 'I'm so tired this moring'. This is wrong since 'i morse' is always past tense - you have to wait for the morning to be over to use it. Johan

I would say that "på sommaren" is useed for every summer. Example "På sommaren är det vanligt att åka till havet för att bada", which means in the summer it is common to go the sea for bathing. Atleast this is the only way to use "på sommaren" I can think of.