CLASS-MON, an add-on for Google Sheets helps teachers identify students' difficulties. It is specially designed for PBL, but can also be used in subjects with other methodologies. The teacher introduces the list of activities that the students must carry out. CLASSMON automatically creates a form with all activities and the following response options: I didn't do the task. I performed the task with many doubts. I performed the task with some doubts. I did the task and I think I got it. When a student finishes an activity, he/she answers the form. The teacher can always see the students' difficulties. He can also detect the most complicated activities. Students can also see their answers and the teacher's assessment.