You can use the fully functional WF demo up to 500 translation units per translation memory (one TU = source segment and target segment). Then you can do either one of three things:
1. Purchase the full license.
2. Create a new TM (you should have separate TM's for different subjects).
3. Use your TM that has 500 TUs as your background translation memory (BTM, cf. manual), create a new TM, and translate up to 500 more TUs. Then purchase the license!