we use une and un before the adj but why do we say 'Marie est etudiante' instead of 'marie est une etudiante' ??????
That's right, however it is ok to say "Marie est une étudiante", it isn't really a huge mistake and lot of French people say it. Be carefull though, if you add an adjective or attribute to the profession you must use "un/une". For example : "Marie est une étudiante française" is correct while "Marie est étudiante française" isn't.
It is just one of the differences between the way the same thing is expressed in different languages. You just have to learn, and accept, the things that make us different.
Why isnt it marie est une etudiante? In French you don't put une or un before the occupation in sentences like this. ... etudiante=female singular etudiant=male singular etudiants=male/mixed plural etudiantes=femals plural the end of words depends on the gender and singularity of the subject