Can someone explain to me the difference between the following forms?
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In English, you might be able to think of those forms like this:
思った ー (I) thought
思っていた ー (I) was thinking, (I) had thought, or (I) had been thinking.
Do note that 思ってた and 思っていた are the same... but 思っていた is more of a form you would want to use in formal writing. In conversational writing, however, you are often free to use either form (depending on the audience to whom the writing is being addressed.)