There are two genders in Swedish: general gender and neuter gender. The indefinite article of general gender is "en" and it is placed before a noun: en flicka, en hund.
The indefinite article of neuter gender is "ett' and it is placed before a noun: ett barn, ett hus.
When add either "en" or "ett" to the end of the noun, this article is considered to be the definite article (becomes the definite article):
en flicka - a girl
flickan - the girl ("e) in "en" is omitted when a noun ends in a vowel)
ett hus - a house
huset - the house (one "t is always omitted)
en vecka - a week
veckan - the week