Explanation

We use Articles A & An before professions.

Doctor gives consonant sound. Therefore, He is a doctor۔  I am a student. She is a teacher. 

کسی کا شعبہ یا فیلڈ بتانے کیلئے ہم (ارٹیکل اے) کا استعمال کرتے ہیں۔

We use an when the profession gives vowel sound.

e.g. She is an actress. You are an actor. I am an author.