Think
When
think means 'believe' or 'have an opinion', we do not use the continuous:
- I think Mary is Canadian, but I'm not sure. (not I'm thinking)
- What do you think of my plan? (= What is your opinion?)
When
think means 'consider', the continuous is possible:
- I'm thinking about what happened. I often think about it.
- Nicky is thinking of giving up her job. (= she is considering it)