1.So what can the root bridge do for the ethernet,what is its main function?i know it is important for the network,and all introductions of STP will teach us how to find it,but they don't why do we need to choose the root bridge from all switches?
A spanning tree has to have a root. Here both terms are like in graphs. A root is a tree node which has no parent node. STP selects this root node as a node with highest bridge id and then constructs shortest path spanning tree rooted at this node. This way one can say that root bridge has a somewhat special role in the topology.
There is no other special feature of the root bridge.
2.The same as the root bridge,korenix also tells us how to find the ring master,but they don't tell us why do we need it,what is the main function of ring master?what can the ring master do for the network?
I can't find any explanation on how redundancy works, but from what i read here ring master has a special role, in that it monitors paths, and presumably decides which direction on the ring the traffic is flowing
Note about root node position
What (R)STP effectively does is to prune a topology (which is a graph) to a tree topology. Nodes can only communicate with each other using tree links. Each path can go either down the tree or first up then down the tree. All other paths in original graph are prunned and not available. For example, a node with highest betweenness centrality could be a good candidate for the root.
Also, in tree routing, nodes higher in the tree are likely to see more traffic. And all paths can potentially go up to the root of the tree. So, you also probably want the root to be more "capable" bridge in the topology. See this question.
Thus position of root note is important with respect to what the resulting calculated paths are and how effective (in comparison to paths in original topology (graph)).