General:
* J'étudie. (Most common and literal translation)
* Je suis en train d'étudier. (More specific, emphasizes that you are studying at this moment)
Specific subjects:
* J'étudie le français. (I am studying French)
* J'étudie les mathématiques. (I am studying mathematics)
* J'étudie la médecine. (I am studying medicine)
Specific context:
* Je suis en train de préparer mes examens. (I am studying for my exams)
* Je suis en train de faire mes devoirs. (I am studying for my homework)
* Je suis en train d'apprendre une nouvelle langue. (I am studying a new language)
You can also use the verb "apprendre" (to learn) in some cases:
* J'apprends le français. (I am learning French)
Remember to use the correct gender and number agreement for the subject you are studying.