"We are girls."
Translation:Vi är flickor.
Yes, there are some rules.
For "en" nouns ending in a vowel:
Words ending in -a change to -or in the plural. Kvinna -> kvinnor. Words ending in -e change to -ar in the plural. Pojke -> pojkar.
For "en" words ending in a consonant, you have to learn the plural. There are some patterns, but with many nouns there are no hard and fast rules and you have to just know it. The easiest is to learn the plural straightaway for every new noun you come across, to drill it in.
One example of a pattern is that "en" nouns ending in -ing always take -ar in the plural, e.g. tidning -> tidningar.
For "ett" nouns, it is simpler and more regular.
If the word ends in a consonant, there is no change. "Barn" means both "child" and "children.
If the word ends in a vowel, add -n to make the plural. Äpple -> Äpplen.