How to add a 2nd monitor to Macbook Air (mid-2012) I have a Macbook Air mid 2012.

If I get a thunderbolt mini display port to HDMI adapter can I connect my macbook to a 2nd monitor and have a certain window (app) running on the main screen while I have a second window (app) running on the connected monitor?
100% yes. Just keep in mind that you can't exceed  a certain resolution (I think it's 4K). Also, buy an hdmi dongle that is compatible (some cheap generic ones offer very poor sound and video quality)

You will be able to use as many apps as you'd like on both monitors simultaneously, and some programs (e.g. Photoshop) even allow you to spread them across multiple monitors.