En and et are indefinite articles and they're used like the english indefinite article "a", but when to use each one of them? Well, nouns that end in -n or-en use en and nouns that end in -t or -et use et and you have to memorise them by heart.
Thanks Michel! I was confused about this myself, your little explanation cleared it right up for me. Tusen Takk! ^_^
Could you please give me the tips link? I've looked for it, but I don't find the norks explanations in the duolinguo webpage. Takk!