Hi everyone. I just completed my first semester as a PhD student in Applied Math at a well ranked US college. I am an international student from an Engineering background, with sufficient programming experience, and my research interests are in numerical fluid dynamics / applied PDE / mathematical physics. I had a perfect score on the general GRE Math, but I wrote the Math GRE and I scored just a little over 600 because I could only really deal with the Calculus problems and then few other sections (most of the material were new to me, since I wasn't a Math / Physics major).

My courses this semester were in Applied Probability, Numerical Analysis and Applied PDE, and I also began some research with my intended supervisor (though it was not required or usual for a first year student me). I haven't done any probability since High School, and this was my first PDE course (didn't take any formally in Engineering though PDEs were encountered), so I had a lot of learning to do in both courses. However, the numerical analysis was mostly a breeze because of my programming background. As the semester progressed I began to pay less attention to the numerical analysis and focus on the other two - thus I lost an opportunity for a relatively easy A in Numerical Analysis due to deliberately uncompleted homework and other time-sacrifices towards the other two courses. I eventually had an A- in Numerical Analysis, a B+ in Applied PDE and a B- in Applied Probability.

I felt I owed the professor an explanation for the B- so I emailed him, and his response was kind of scary, talking about how I "have to discuss my future as a grad student in the department" based on my poor showing in the probability class. As far as I know, this is the final probability class I will be taking. Should I be concerned? Plus, I once read here that grades don't matter that much in grad school if you are able to hold your own in research (which I believe I can). Any advice will be very welcome, please.