How much is expected to know before starting a PhD I recently graduated with a BSc in applied mathematics and considering on applying for a PhD. In fact, I contacted a supervisor and he seems some what interested in my undergraduate academic performance. He also sent some articles to read so that I get an idea of what sort of problem is there to be analysed in the PhD, and of possible methods to follow. I read them, however I do not understand everything in those papers as I am not familiar with some of the methods used. However, as I have a basic background knowledge from undergraduate coursework, I believe, I can study those methods in depth and gain knowledge. But it would obviously take couple of months for me to learn the material. Although my PhD wouldn't start (if I get selected) for another 6 months, I have to tell the potential supervisor about what I read in the articles he sent and how much I understand.   

So, if I say like I didn't understand all the details, but I intend to learn them before the start of the PhD, does it look negative? Does supervisors expect us to know everything before the commencement of the research?
The best way to respond to this specific question is to engage the potential supervisor about specific content in the articles.  You are not expected to know everything at this stage.  But it is important to show you are interested, can engage with the material, and (not least) actually took the time to read what they sent.  

A response like "I read them but I didn't understand everything" doesn't give them any information about you.

Be specific, e.g.


I found the way authors applied X method in this paper to be really interesting.  I was familiar with this but didn't realize it could be used this way.
The authors did X but why didn't they use alternative approach Y?
I don't understand what this statement means...could you explain it?


Asking questions about things you don't understand can be one of the best ways to engage.