What are the difference between e.g. 想, 觉得 and 认为 when expressing opinion?
I believe it's the same as in English:  


  我想 - I think (that) xxxxxxxx
  我相信 - I believe (that) xxxxxxx
  我觉得 -
  I feel (that) xxxxx


I will hold to explain how to use each of these in English as you should have an idea already.