The word is 'thought.' But, I would say that only the inverted form is commonly used. For example: 'She was a very nice girl, or so I thought.' The meaning of the above is that you thought she was nice but that you later found out that she wasn't really.

Kurallar sabit değil, ancak 1) "so I thought" > Düşündüm (ama yanlıştı) 2) "I thought so" > Düşündüm (ve gerçek oldu)