adding 시 before you conjugate it makes it more polite

I believe politeness but im not sure. Get out the ole googler and see what it says

Adding the 시 in is a way to express respect for the person that you're talking about (in this case the teacher/doctor/etc.) where as the -ㅂ니다 bit is for respect to who you're talking to (a coworker for example).

The latter is a "honorific" form of the former.