"I am a child."
Translation:Jag är ett barn.
Almost all living things are en words in Swedish. Ett barn, ett lejon (a lion) and ett får (a sheep) are the only living things i can think of off the top of my head that are ett words.
Most normal objects are en words too, but if you're doing living things you can just assume that and animals and people are en words except for those three.
As for the objects you just have to learn them by heart, though if you find them in the wild its very easy to tell if a noun is en or ett because there's almost always one more word in the sentence that will give it away.