Dear Mr. (name),
I am writing in connection with your paper "(article title)" in (journal name) which I have recently read.
I am very interested in this area of research and would like to ask some questions relating to your paper. I realise that you must be very busy, but I hope that you are able to find the time to answer them. They are as follows.
1. What ~~~?
2. How ~~~?
Even if, owing to pressure of work, you are unable to respond at this moment in time. I would like to thank you for your time and wish you well in your future endeavors.
Student or Learner