Why is there not an indefinite article before the word for girl?
When you talk about your sex or your profession you do not use articles. Examples:
Eu sunt inginer. - I am an engineer.
Ea este profesoară. - She is a teacher.
Ei sunt bărbați. - They are men.
Prietena mea este studentă. - My (girl)friend is a student.
Just like with the German language. :)
Only with proffesions, it doesn't work with boy/girl/woman etc.