'Part time' and 'part-time' are the same thing - they are used as an adjective - 'a part-time worker'. If you look over the internet, the preferred form is 'part-time'. I'm trying to think where 'part time' would be used other than an adjective and I can't. 'Easy-to-use' is an adjective, where 'easy it use' is a phrase. They are indeed different. "This is an easy-to-use tool" (adjective) "This tool is easy to use." (phrase) Both the above uses are correct.

The word part-time is related to working. When one works less than 40 hours per week in the U.S.A. this is called part-time work. The term easy- to- use can be used to explain qualities of something like a tool or manual. For example: The program comes with an easy- to- use format. The hyphen is used in some compounds, especially those containing prepositions. For example: He has a devil-may-care attitude. It is an over- the- counter drug. To use the words part time without a hyphen you would have to say for example: He uses the truck to drive to work part of the time. The tool that he uses is very easy to use for digging in the garden.