How to handle an advisor who regularly gives me new work for my thesis? I'm currently working on my master thesis and will have to hand it in in April. Of course, the topic of my thesis is somehow restricted to a certain problem, but my advisor keeps telling me new problems to consider for my thesis, which have little relation to my original master thesis problem. 

The problems he gives to me are very interesing, but they do not fit into my master thesis and if I had to include them, I would get into serious time troubles. I talked with him about this and he always answers very vaguely, e.g. with "Let's see how your thesis turns out to be". His answers are also very mood-dependent, so one has to be careful.

How can I convince him that his problems are not fitting for my thesis and that it is simply too much? I have to hand my thesis in soon and now, at this point, he comes up with new stuff to include. Furthermore, I am afraid of getting a bad grade if I don't do what he tells me to do. I am planning to start my PhD study under his supervision afterwards. Could I perhaps propose that I work on his problems after my master thesis?

Thanks in advance, I appreciate any sort of advice.
I am planning to start my PhD study under his supervision afterwards.


Is he aware of that? If not, you should probably consider what the PhD workload would be, if you are already experiencing the pressure you describe with your master thesis. If he is aware of it, it is possible that he is trying to broaden your views in order to prepare you more for the PhD work and probably being a bit overzealous by doing so.

However, in both cases you shouldn't be afraid to talk to him about where to draw the line for your thesis. That is also an important part of graduation. Personally, I don't take much into account the "mood" of my colleagues when discussing important professional subjects. If you adviser is so unstable, that an objective discussion when he is in one of his moods is not possible, that is another and more serious problem. In that case, you should consider finishing up your masters and going to grad school somewhere else, or at least with another adviser.

As for what you should do, as I said, approach him about your problem and determine where you are with your work and what is the scope of your thesis. It is normal to draw the line somewhere and proclaim that that amount of work will constitute your thesis. Once that is down, you stick to it. As I understand it, you have your problem stated, so you can any other proposed problems delay with "My thesis is my priority right now, but I'll keep that particular in mind, and once I've finished my thesis, we can start working on that one."