It's hard to explain. I don't like the first sentence at all, by the way, it seems disjointed. Compulsory and mandatory essentially mean the same thing, as in you have no choice. But compulsory means you have to do, mandatory means it must be done. There's little difference between them and I'm struggling to explain it. Mandatory means you have to follow some rules, compulsory tends to have a feeling of something not yet being started or finished, so it's compulsory that you must do something, because they are the rules, and also it's one thing related to another, like if you're 16 in a country where they have national service, it's compulsory for you to serve in the Army for 4 years. And National Service is mandatory. So compulsory is a relative insistence Mandatory is an overall rule which is like a law in itself. I'm sorry it's hard to answer. You can sometimes swap one word for another, but not in this case. The first sentence is detailing the age range, and mentioning service being compulsory. The second sentence is saying National Service is mandatory. National service is mandatory, if you're aged between 18-33 it's compulsory. If a person asked, "will you have to do National Service?" A person could answer, "yes it's compulsory for people aged 18-33" or "yes it's mandatory" (and assume you KNOW the age range and know they fall within it.) So the words in this case are not strictly interchangeable. If a person was to write, "National Service is mandatory for people aged 18-33" they would be slightly inaccurate, National Service is mandatory, and it's compulsary for people aged 18-33. So the law that people have to go, stands. The people the law affect are 18-33 for whom it's compulsory to join. Perhaps I'm "splitting hairs" but I don't think so, this is how you get to be C2 :D haha :D