"Wearing dress shoes is uncomfortable."
Translation:구두를 신는 것은 불편해.
Overall, using 이/가 indicates that you found something out by yourself, by experience (eg. you are wearing these dress shoes right now and you are feeling uncomfortable).
은/는 on the other hand provides the feeling of comparing (when used specially for comparing there should be an another sentence to have enough context. Here, as additional information it could be: "my old sneakers are so comfortable").
But hey, don't worry about that at the beginnig - I heard that Koreans write their scientific works about the differences between those particles :) So you can basically use 이/가 and 은/는 interchangeably.