·  To express opinion you can use the following expressions: I think, I
   believe, I guess, I don’t think, I don’t believe, I suppose, In my opinion, in
   my view.

·  To say that you are not sure but something is possible you can use words
   like “may”, “might”, “should”, “could”, “perhaps”, “possibly”.

·  To show that your ideas are based on some facts you can use 
   expressions like: “based on the fact that…”, “according to…”, “in
   accordance with…”, “judging from…”, “due to the fact that…”.