"We swim in the summers."
Translation:Vi simmar på somrarna.
i and på are the trickiest ones, this ^^ might help :P
<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.