"Think" alone is present tense. "I think Bob may be a vampire". "Have thought" refers to the past, in a form called "past progressive", which refers to an ongoing or repeated event during the past that continues into the present. "Several times over our friendship, I have thought that Bob was unusually interesting in my neck." There are other forms as well, such as simple past ("Yesterday I thought I saw Bob turn into a bat") and past perfect, or complete past ("I had thought Bob was a vampire, but later I learned he is a werewolf.")

think is present, have thought is past tense. "Right now, I think that....." "I have thought about that in the past, but I decided against it"

Hi Ei🤗, so thinking is a manual process, when you're answering a question you're thinking what you're going to say. While thoughts are just random things that pop ups in your brain, just like when you're spacing out.