In the past, I had a lot of difficulties to understand the difference between liberal and progressive. In fact, I was convinced, and actively used the word progressive as a synonym for the word liberal. In the book The Conscience of a Liberal, Paul Krugman states that the difference between liberal person and progressive person is similar to the difference between a philosophy and taking action. That is, while being liberal implies that a person believes that an increase of the unionization of workers will reduce inequality, being progressive means actively increasing unionization because one believes that it decreases inequality.

References

Krugman, P. (2009). The conscience of a liberal. WW Norton & Company.