Simple tense - I think

If we want to express our opinion or belief, we don't usually use the continuous.

We can express this in the past, present and future.

Past simple:

• "He thought the presentation could be better."

Present simple:

• "I think that's a fantastic idea."

Future:

• "What will she think about it?"

Continuous tense - I'm thinking

If we want to expression consideration or contemplation, it is preferable to use the continuous.

We can also express this in the past, present and future.

Past continuous:

• "He was thinking of changing careers."

Present continuous:

• "She is thinking of resigning from her job."

Present perfect continuous:

• "I have been thinking about it."

Future continuous:

• "I will be thinking about it."

Some exceptions

Both "thinking" and "think" can mean "to contemplate" or "to reflect on."

• "I've been thinking about it, I think about it all the time."

In the present perfect, it is more likely that it's about consideration rather than opinion.

• "I have thought about it for a while."