Here's how it works:
* Think is the present tense of the verb.
* Thought is the past tense and past participle of the verb.
* Thinking is the present participle of the verb.
For example:
* I think about my future. (present tense)
* I thought about my future yesterday. (past tense)
* I have thought about my future many times. (past participle)
* I am thinking about my future right now. (present participle)