how to use a .ucf file?

Hello all, I understand that I can assign timing constraints using a .ucf file. I tried defining one (using the wizard) and including it in the project. In synthesis properties it is not possible to point out a .ucf file, and a .xcf file is expected. I left this blank. I selected the option 'write timing constraints file' and run the synthesis process. I see the tool consulting the .ucf file and gives no warnings. I ran Implement design processes and have a information that all of the timing constraints are met. When I run the simulation using ModelSim, .sdf and .vhd file generated by the ISE tool, the results look like the timing constraints are not met (even if I select a frequency much smaller than the tool accepted). Hints? Thanks in advance, Marija