Can I use these both interchangeably?
Or so I tought.
Or I tought so.

Question

Answer

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.

Answer

Kurallar sabit değil, ancak

1) "so I thought" > Düşündüm (ama yanlıştı)

2) "I thought so" > Düşündüm (ve gerçek oldu)