I do not know how to use this word: upon. 

Whenever I saw this word, i can not understand it with other words. 

Thanks for your help.

And I have another question. When I am writing a article to explain some points from myself. I need to use personal pronoun. Which I should to use: you or they? or other personal pronoun, For example, I want to  explain students need study alone to get more time to go through difficult problems or abstract concepts.. I can say that:

1. They need study alone to get more time to go through difficult problems or abstract concepts.

2. You need study alone to get more time to go through difficult problems or abstract concepts.

Which sentence is better to explain my opinion. 

Thanks.