I don’t really understand the use of tal
I tend to think of ‘tal’ as ‘such’. A word-for-word translation of the sentence would be “It was such as I thought”. With a bit of a nudge this becomes “It was just like I thought”.
It’s one of those words that doesn’t have a one-to-one match in English, so it necessary to get a feel for it various uses.
why isn’t “pensaba” accepted?
It was just as I thought.
Would it make sense without the “tal”? “Era como yo pensé”?
Yes, but tal makes it broader - “It was about what I thought”