[Roosh]

How do I say something like, "I thought I had to go, but I didn't."

I'm thinking... "Pensé/Creí que tuviera que ir, pero no."

Also I see "I thought" showing up both in preterito and imperfecto. Kind of confusing as to which to use. I figure if I want to say "I used to believe" it's in imperfecto and if I want to say "At that moment in time I thought" it's preterito.

[lilactag]

How do I say something like, "I thought I had to go, but I didn't."
Creí que tenía que irme, pero no lo hice

I'm thinking... "Pensé/Creí que tuviera que ir, pero no."

Also I see "I thought" showing up both in preterito and imperfecto. Kind of confusing as to which to use. I figure if I want to say "I used to believe" it's in imperfecto and if I want to say "At that moment in time I thought" it's preterito.

Your understanding of the use of creí/creía seems correct to me
i.e
"I thought you knew i was coming with you"
Yo creí que tu sabías que yo (iba a venir) / vendría contigo

"I used to believe in God"
Yo creía en Dios

"I believed in your word"
Yo creí en tu palabra

Hope it helps, i am getting confused myself by now hehe

[vicen]

that's right