"A" and "an" in English depend on the next letter, not the next word. You can have "an elephant" but then "a big elephant", however in Danish it will be "en elefant" and "en stor elefant". In the same way in English, you can say "a house" but "an impressive house", but in Danish it's "et hus" and "et imponerende hus".
Danish has two genders for nouns and you have to learn the word with the gender (instead of learning "house = hus" learn "a house = et hus", for example). 75% of all nouns are common gender (n-words) while the rest are neuter (t-words). There is no real patterns other than some noun-endings always having the same gender, and in compound nouns, the gender of the noun is pretty much always the gender of the final word.