Activity UML

Hi am a newbie to UML

i need help on an exercise am working on, i created an activity diagram to reflect a student enrolling into a specific university course and As part of the “Validate Program of Study” functionality, the system is required to check for timetable clashes. For each new course offering a student wishes to take, a check must be done on the existing course offerings to make sure the new offering does not occur at the same time. i have done quiet a lot of documentation but the part where is need to add validate and check timetable clash is where i need help. Any ideas team.

Thank you in advance