Which and how much we should learn?

Which and how much we should learn?

After being a college teacher for one year, I began to pursue my docoterate degree. Due to my research orientation, much mathmatical knowlege is needed, such as linear algebra, calculous,matrix theory,geometry ect. Many other subject also needed, for example mechanics, information theory ,even art  and so on.  Here I want to say that, when we were in some stage of study( such as being a freshman or higher in university ) we have no clear idea which subject is import to our major or what the purpurse of studying this subject. As an experienced person, i hope my advice is useful to you. No matter whether you know the importance of some subject, you should learn it by heart and try to study deeper, for some day you will find it is so  important to your research or study. When you need some knowledge that you ever had but didn't pay enough attention to it, you will be in a rough condition, because you dont have that time to study it.  If you bury your head into a book you will find so much need studying and so much is out of your imagination. With the accumulation of the learned knowledge, you will find the inner connection with each other and your aim is clear then.