Welcome! This is a common question here. I just did a quick search on bikeforums, and it is hard to sift through all the results. The short answer is that a used, non-suspension mountain bike makes an excellent short to mid distance urban commuter, especially with smooth, street tires. But it is worth your while to try searching and reading through this forum for tips that fit your commuting needs. Although I have been bike commuting in one way or another my whole life, and in earnest sonce the early 1990s, when I found bikeforums, things got real good, real fast bike-commuting wise.