In your example, you say "this is a pen" when it is close to you, (perhaps it is in your hand), you can see it at the time of speaking. and you want to introduce it into the conversation. "[I am speaking to a new student] Look! This is a pen (in my hand). I will use it to mark your homework."

Hi Tom! Thanks for prompt reply. This is a pen and It is a pen. Could you please clarify the difference here?

Hi Maria, They can normally be used interchangeably when talking about an object, but sometimes there are some subtle differences: Eg: "What is this!" can be used when people enter a room and find others doing things they should not! Eg: "What is it?" can be used by a person talking to another to see how they are (like when a child comes to a mother perhaps crying and the mother asks "What is it?" All the best, Tom