formal そうだと思ってました。 informal そうだと思った。 or　やっぱり(ね) やっぱり is a spoken language, and a very casual phrase. やっぱり means "That’s what I thought" "I thought so" "I knew it." I think you can use やっぱり when you talk with your friends and someone close to you. Some young Japanese say やっぱ(ね), and some Japanese say やっぱし(ね).

I'd like to know the answer to this as well!

やっぱりな