You should use this word depending on the desired time. Think - is present time Thought - is past time

'thought' will either indicate the past tense, the action of thinking happening in the past : "When I was young, I thought, that money was the most important thing in life." or you could use it in a present time at the moment to refer to uncertainty,when you expect or supose something ,but you are not sure of it: "I thought ,you would like to come with us." "think" in the present tense always refers to a present action of thinking about or of something. "I think, I will have to call her now."

I always thought I am very smart, but now I think I am wrong in this case.

i THINK you have understood this example.THINK =present simple tense I THOUGHT you had not learnt your lesson=thought= past simple tense and this is irregular verb.