assertive situations

Today we talked about different situations, when we should be assertive. Firstly, we had to envision the last situation, when we were criticized. I just remembered, when last time during an experiment of my physics class my teacher criticized the way, how I do it, but he showed me, how I can do it well, so I think it was a really assertive conversation between us. But as a conclusion of this task, we realized again, that many things depend on the details of the situation (like the other person, the mood of you/the other person, etc...). After that we solved some situations, when it is not easy to be assertive. I chose the situation, where one of my friends got money from me, but he/she has forgotten it and doesn't pay it back. In a situation like this, at first I made him/her remember the moment, when I gave him/her the money, then I would say that I need it back. But for example if I knew, that my friend is not able to pay it back, because of financial problems, I wouldn't ask him paying back, so all in all the solution always depends on a lot of detail.