• Think can mean ‘have an opinion’. In this meaning, it is not used in progressive tenses.
      I don’t think much of his latest book.
      Who do you think will win the election?
  • When think has other meanings (for example plan, or consider) progressive tenses are possible.
      I’m thinking of changing my job.
      What are you thinking about?
  • When think is used to introduce a negative idea, we usually construct the sentence I do not think . . ., not I think . . . not .. .
      I don’t think it will rain.
      Mary doesn’t think she can come.