"We swim in the summers."

Translation: Vi simmar på somrarna.

[jhlouise]

Sorry if this is a silly question, but how do you figure out whether to use i/på/etc?

[Arnauti]

For starters:
Days of the week: på - på måndag - on Monday
Months: i - i augusti - in August
Seasons: på - på sommaren - in the summer

[DehPuh]

Either you practice a lot or you learn all the cases by heart. I suggest practicing.

[jhlouise]

I can usually make an educated guess, but sometimes it confuses me - I was just wondering whether there was any general rule I could follow. Thanks anyway! :)

[DehPuh]

http://blogs.transparent.com/swedish/swedish-prepositions/

i and på are the trickiest ones, this ^^ might help :P

[DehPuh]

why does sommar lose an M? is mmr a no-no cluster? which other words behave alike? I'm so full of questions right now

[Blehg]

<m> is only ever doubled if it is between vowels (or in a few cases at the end of a word following a vowel). Otherwise it's always on it's own.

sommar - somrar
hammare - hamra 'hammer - to hammer'
dum - dummare 'dumb - dumber'
fem - femma 'five - a number five'

It's actually a very stupid rule because it doesn't comply with the rest of the system. If the consonant is single, the preceding vowel is supposed to be long - but not <m>. dum, vem and döma all have short vowels.

[Jan-Olav]

In some cases double m is used in the end of a word when there is another word with just one m, for instance lam (lame) - lamm (lamb), dam (lady) - damm (dust/pond).

[Zmrzlina]

Yes, mmra is a no-no. The word nummer behaves the same, being numret and numren in definite singular and definite plural respectively. I don't remember other cases right now, but the rule is that mmra is awkward unless it loses an M.

[friswing]

dubbel -mm- or -nn- are always between vowels, and always becomes singel -m- or -n- when followed by a consonant (inside the same word).